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Maths help: Reverse percentages and standard index form

Please help me! I'm literally the worst at maths...

Reverse percentages question:

If, two years ago, David Coulthard had to pull all his Formula One earnings into a bank account and earned 5% compound interest, he would now have $7,938,000 in his account. How much did he earn two years ago?


Standard index form question:

A factory produces 3.6 x 10^6 nails each year. Each nail has a mass of 5 x 10^-3kg. Of all nails produced, 0.75% are faulty. Work out the total mass of all the faulty nails produced in one year.
I have to ask because we can't give full solutions, but what have you tried? If you literally don't know where to start I can give hints.
Reply 2
Original post by TheIrrational
I have to ask because we can't give full solutions, but what have you tried? If you literally don't know where to start I can give hints.


Wait, why can you give full solutions?? This isn't homework btw, I'm revising off a workbook for my maths exam coming up soon...

Well for the reverse percentages question I tried finding 5% of it which I got as $756,000. I then subtracted that from $7,938,000 which left me with $7,182,000. I then tried to find 5% of this reduced number because that's what you do with compound interests right? Anyway this is the bit where I started to get confused...

For the second question... I have no clue on what they're asking. Literally sounds gibberish to me
Original post by hotmom
Wait, why can you give full solutions?? This isn't homework btw, I'm revising off a workbook for my maths exam coming up soon...

Well I for the reverse percentages I tried finding 5% of it which I got as $756,000. I then subtracted that from $7,938,000 which left me with $7,182,000. I then tried to find 5% of this reduced number because that's what you do with compound interests right? Anyway this is the bit where I started to get confused...

For the second question... I have no clue on what they're asking. Literally sounds gibberish to me


We can't really give full solutions because it could be homework and it won't help some people learn properly if we just write it all out.

For 1)

call the money he had before the interest A, the you know that after the 2 years compound interest of 5% he will have $7,938,000. So writing this in maths you have AΓ—1.052=7,938,000A \times 1.05^2 = 7,938,000. Then you can rearrange this to find A, the amount of money he had 2 years ago.

For 2)

Now you know the company produces 3.6 x 10^6 nails each year, and 0.75% are faulty, now 0.75% =0.0075=7.5Γ—10βˆ’3= 0.0075 = 7.5 \times 10^{-3}, so how many of these 3.6 x 10^6 nails are faulty each year? Then if each one weighs 5Γ—10βˆ’35 \times 10^{-3}kg, then how much do all the faulty ones weigh?
(edited 8 years ago)
Original post by hotmom
Wait, why can you give full solutions?? This isn't homework btw, I'm revising off a workbook for my maths exam coming up soon...

Well for the reverse percentages question I tried finding 5% of it which I got as $756,000. I then subtracted that from $7,938,000 which left me with $7,182,000. I then tried to find 5% of this reduced number because that's what you do with compound interests right? Anyway this is the bit where I started to get confused...

For the second question... I have no clue on what they're asking. Literally sounds gibberish to me


Okay so when you calculate compound interest the normal way (not in reverse), if he put in 100 at 5% interest than he would have 100Γ—(1.05)2=110.25 100 \times (1.05)^2 = 110.25
Therefore if you want to do it in reverse you must divide by (1.05)2 (1.05)^2

So can you think what it would be in this situation?
(edited 8 years ago)
OP,I would suggest you use Mathswatch.
Reply 6
Original post by German123
OP,I would suggest you use Mathswatch.


I don't have a login because my school doesn't use it :frown:
Original post by hotmom
I don't have a login because my school doesn't use it :frown:


Ouch.

Well, there are other online website you can use like Hegarty maths and Corbett maths, these are good and there are tons more.

The ones listed are the ones I used.:smile:
Reply 8
Original post by german123
ouch.

Well, there are other online website you can use like hegarty maths and corbett maths, these are good and there are tons more.

The ones listed are the ones i used.:smile:



thank you!! 😊😊
Original post by hotmom
thank you!! 😊😊


:smile:

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