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# How do you work out these maths calculations? watch

1. Can someone help me with these, how do I work them out?

1. 6 x ? - 35 + 75 = 340

2. 216 ÷ ? x 6 = 162

3. ? x 15 + 105 - 5 = 400

4. 120 x ? ÷ 3 = 160

5. ? ÷ 7 + 54 - 42 = 57

6. ? x 3 ÷ 4 - 5 = 7

7. 720 ÷ 12 - ? + 313 = 330

8. 480 x ? ÷ 4 + 50 = 290

9. ? x 35 - 107 = 768

10. 528 x 6 ÷ 6 + ? = 584

11. 420 ÷ 7 x ? - 50 = 850
2. Just rearrange them

I.e. 6x?-35+75=340

? = 300/6

? = 50

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3. (Original post by unfathomablee)
Just rearrange them

I.e. 6x?-35+75=340

? = 300/6

? = 50

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How/Where are you getting the 300 from?
4. –35 + 75 = 40

So 6 x ? – 35 + 75 = 340 is the same as 6 x ? + 40 = 340

Move the 40 to the other side...

6 x ? = 300

Divide both sides by 6...

? = 300/6 = 50
5. Do you know how to do algebra with x and y?
6. (Original post by efn)
Can someone help me with these, how do I work them out?

1. 6 x ? - 35 + 75 = 340
Both sides of the = sign are the same.

So what you do on one side, you need to do to the other to maintain balance.

6 x ? - 35 + 75 = 340

Take 75 away from the left and right hand sides.

6 x ? -35 + 75 - 75 = 340 -75

+ 75 - 75 on the lhs is zero so the sum becomes:

6 x ? -35 = 340 - 75

6 x ? - 35 = 265

Now add 35 to the lhs and do the same to the rhs

6 x ? - 35 + 35 = 265 + 35

once again -35 + 35 = 0 and the sum becomes

6 X ? = 300

Now divide by 6 on both the left and right so the some becomes

(6 x ?) ÷ 6 = 300 ÷ 6

6 ÷ 6 on the lhs = 1 so the sum becomes

1 x ? = 300 ÷ 6

and 1 x anything is left unchanged, so....

? = 300 ÷ 6

? = 50

The idea is to eliminate the numbers on the left one by one such that the only remaining thing is the unknown number. i.e. ?
7. (Original post by uberteknik)
Both sides of the = sign are the same.

So what you do on one side, you need to do to the other to maintain balance.

6 x ? - 35 + 75 = 340

Take 75 away from the left and right hand sides.

6 x ? -35 + 75 - 75 = 340 -75

+ 75 - 75 on the lhs is zero so the sum becomes:

6 x ? -35 = 340 - 75

6 x ? - 35 = 265

Now add 35 to the lhs and do the same to the rhs

6 x ? - 35 + 35 = 265 + 35

once again -35 + 35 = 0 and the sum becomes

6 X ? = 300

Now divide by 6 on both the left and right so the some becomes

(6 x ?) ÷ 6 = 300 ÷ 6

6 ÷ 6 on the lhs = 1 so the sum becomes

1 x ? = 300 ÷ 6

and 1 x anything is left unchanged, so....

? = 300 ÷ 6

? = 50

The idea is to eliminate the numbers on the left one by one such that the only remaining thing is the unknown number. i.e. ?
Do you always have to minus the largest number and do the same to the other side of the equation? What about some of the other examples including division?
8. (Original post by efn)
Do you always have to minus the largest number and do the same to the other side of the equation? What about some of the other examples including division?
No, you can do either (for the + or - signs) but you must ALWAYS do the same on both sides or the equality sign (=) is meaningless. Think of it like a see-saw perfectly balanced. you need to keep it perfectly balanced at all times so taking away weight form one side must be accompanied by taking the same weight away from the other to keep the balance.

There are rules you need to follow for algebra and you may be aware of

BODMAS ?

http://www.mathsisfun.com/operation-order-bodmas.html

This means that the order of carrying out operations is critical.

When working out equations with a missing number, use BODMAS in reverse.

i.e.

SAMDOB
9. (Original post by efn)
Can someone help me with these, how do I work them out?

2. 216 ÷ ? x 6 = 162
216 ÷ ? x 6 = 162

216 ÷ ? x 6 ÷ 6 = 162 ÷ 6

216 ÷ ? = 27

216 ÷ ? ÷ 27 = 27 ÷ 27

8 ÷ ? = 1

? = 8
10. (Original post by uberteknik)
No, you can do either (for the + or - signs) but you must ALWAYS do the same on both sides or the equality sign (=) is meaningless. Think of it like a see-saw perfectly balanced. you need to keep it perfectly balanced at all times so taking away weight form one side must be accompanied by taking the same weight away from the other to keep the balance.

There are rules you need to follow for algebra and you may be aware of

BODMAS ?

http://www.mathsisfun.com/operation-order-bodmas.html

This means that the order of carrying out operations is critical.

When working out equations with a missing number, use BODMAS in reverse.

i.e.

SAMDOB
Ah ok

So here ? x 15 + 105 - 5 = 400

We have to apply subtraction/addition first, so this would be : ? x 15 + 105 - 5 +5 = 400 + 5 ?
11. (Original post by efn)
Ah ok

So here ? x 15 + 105 - 5 = 400

We have to apply subtraction/addition first, so this would be : ? x 15 + 105 - 5 +5 = 400 + 5 ?
Yes. That is correct.

Although a slightly quicker way is to spot that you can do the sum 105 - 5 = 100 first without affecting the right hand side.

? x 15 + 100 = 400

Try them both and you should still get the same answer at the end.
12. (Original post by efn)
Can someone help me with these, how do I work them out?

1. 6 x ? - 35 + 75 = 340

2. 216 ÷ ? x 6 = 162

3. ? x 15 + 105 - 5 = 400

4. 120 x ? ÷ 3 = 160

5. ? ÷ 7 + 54 - 42 = 57

6. ? x 3 ÷ 4 - 5 = 7

7. 720 ÷ 12 - ? + 313 = 330

8. 480 x ? ÷ 4 + 50 = 290

9. ? x 35 - 107 = 768

10. 528 x 6 ÷ 6 + ? = 584

11. 420 ÷ 7 x ? - 50 = 850
Always use the rearranging formula, changing the subject kind of.
If you want to bring +75 to the other side it becomes - 75.
I hope you understand what i mean,

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Updated: December 28, 2014
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