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# Formulae / Changing the subject watch

1. Having trouble working this out..

Q - The fraction, (p), of an adults dose of medicine which should be given to a child who weighs, (w)kg, is given by the formula - p= 3w+20/200

a. Use the formula to find the weight of a child wose dose is the same as an adult..

Help..
2. (Original post by Bryz)
Having trouble working this out..

Q - The fraction, (p), of an adults dose of medicine which should be given to a child who weighs, (w)kg, is given by the formula - p= 3w+20/200

a. Use the formula to find the weight of a child wose dose is the same as an adult..

Help..
So what fraction do you need if the doses are the same

That gives you p

Solve for w
3. (Original post by TenOfThem)
So what fraction do you need if the doses are the same

That gives you p

Solve for w
sorry, i still don't understand

i am poor at maths..
4. (Original post by Bryz)
sorry, i still don't understand

i am poor at maths..
P is the fraction

If a child needed half an adult dose p would be 0.5

The child needs a full adult dose, so what is p
5. (Original post by TenOfThem)
P is the fraction

If a child needed half an adult dose p would be 0.5

The child needs a full adult dose, so what is p
Just worked it out, thanks, p = 60

BUT the next question, I was asked to make w the subject, and I am stuck halfway through it, could you please make w the subject so i can see where i am going wrong?
6. p= 3w+20/200

Subtract 20/200 from both sides and divide by 3

20/200 = 1/10 or 0.1
7. (Original post by L'Evil Fish)
p= 3w+20/200

Subtract 20/200 from both sides and divide by 3

20/200 = 1/10 or 0.1
Bit late mate Thanks anyways!

BUT the next question, I was asked to make w the subject, and I am stuck halfway through it, could you please make w the subject so i can see where i am going wrong?
8. (Original post by Bryz)
Bit late mate Thanks anyways!

BUT the next question, I was asked to make w the subject, and I am stuck halfway through it, could you please make w the subject so i can see where i am going wrong?
Can't see anything?
9. (Original post by L'Evil Fish)
Can't see anything?
What do you mean?

I was asked to make (w) the subject of the original equation : p = 3w + 20/200

I did most of it, but I am stuck on the last bit or have done it wrong, could you please do that with your working so I can see if I am correct/where I went wrong?
10. (Original post by Bryz)
What do you mean?

I was asked to make (w) the subject of the original equation : p = 3w + 20/200

I did most of it, but I am stuck on the last bit or have done it wrong, could you please do that with your working so I can see if I am correct/where I went wrong?
I told you
Subtract 20/200 from both sides.
Divide by 3.
11. (Original post by L'Evil Fish)
I told you
Subtract 20/200 from both sides.
Divide by 3.
How does that make sence?:O

Could you show each step logically, and represent both sides please? Thanks

Also the next question after that is to :

Express A in terms of W in the following formula

3w+20/200 = A/A+12
12. (Original post by Bryz)
Just worked it out, thanks, p = 60

BUT the next question, I was asked to make w the subject, and I am stuck halfway through it, could you please make w the subject so i can see where i am going wrong?
No

P does not equal 60

Btw

You should have said p=(3w+20)/200
13. (Original post by Bryz)
How does that make sence?

But, as I say, it isn't

What do you do to w to get p

Do the reverse to p to get w
15. (Original post by TenOfThem)
No

P does not equal 60

Btw

You should have said p=(3w+20)/200
no.wonder...
(Original post by Bryz)
How does that make sence?:O

Could you show each step logically, and represent both sides please? Thanks

Also the next question after that is to :

Express A in terms of W in the following formula

3w+20/200 = A/A+12
The first:

Multiply by 200

Now subtract 20 from both sides and divide by 3.
16. (Original post by L'Evil Fish)
no.wonder...

The first:

Multiply by 200

Now subtract 20 from both sides and divide by 3.
Thank you my friend!
17. (Original post by TenOfThem)
No

P does not equal 60

Btw

You should have said p=(3w+20)/200
Sorry!

Could you guys show each step logically for this question? :

Express A in terms of W in the following formula

3w+20/200 = A/A+12
18. (Original post by Bryz)
Sorry!

Could you guys show each step logically for this question? :

Express A in terms of W in the following formula

3w+20/200 = A/A+12
So, even though you are sorry, you are not willing to use the bracket keys to ask this question
19. (Original post by Bryz)
Sorry!

Could you guys show each step logically for this question? :

Express A in terms of W in the following formula

3w+20/200 = A/A+12

(3w+20)/20 = A/(A+12)

Make.a.common denominator? Or multiply throughout by 20(a+12)

What do you get?
20. (Original post by TenOfThem)
So, even though you are sorry, you are not willing to use the bracket keys to ask this question
Ok, from now on I shall use brackets! That statement you made did make me chuckle though.
(Original post by L'Evil Fish)

(3w+20)/20 = A/(A+12)

Make.a.common denominator? Or multiply throughout by 20(a+12)

What do you get?
You've lost me..

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