can you please help me.., with this math problem? Options

[KERP1UNK]

can you guys please help me with this math problem?? thx ;D

At a fixed interest rate, an investment of $8,000 earns $420. How much do you need to invest at the same rate to earn $672?

i have no idea how to do this.. :(

[starlette]

ooh fun a proportion!! I was soo good at these. okay you have the investment 8000 over a rate of 420 so thats 8000/420 and you have investement we need to find so thats x to get the rate 672 so thats x/672 then you cross multiply. Do you know how to cross multiply? Its where you take the top number on the first, times the bottom number on the other, and put it on top of a new fraction, then take the bottom number on the first and multiply it by the top number on the second one and out it on the bottom of your fraction. I won't give you the answer, but heres an example so you can find out how to work the problem.

An easy one.

Sally worked 5 hours yesterday and made 50 dollars. How many would she have to work to make 100 dollars?

U have your first fraction, 5 hours over 50 dollars (5/50), and your second fraction which is the number of hours again (x) over 100 dollars. (x/100) so now you cross multiply. You have 5/50 x X/100 = ? multply the 5 by 100 you get 500, so that goes on top of the fraction (500/ ) and then you multiply 50 by x and you get 50x for the bottom number making it 5/50 x X/100 = 500/50X. Now what you will do is solve for x. So to solve for x you would take 500 and divide it by 50 (500/50=x) and 500 divided by 50 is ten, so x=10. There. I know thats a lot, but basically, plug in your numbers and do the math. Its really very simple. These are my favorite kind of numbers. If you still cant get it, pm me and i will try to explain it further.

umm..also there is another way. Everything is the same except for how I explained the cross multiplication. Do the same and cross multiply your 5 and 100 for 500, and put it on one side of an equals sign (500= ) and then take your 50x and put it on the other side so you get 500 = 50x. Now to solve for x you divide both sides by 50, which cancels out the first 50 (50 devided by 50 is one, so you have 1x left, which is redundant.) and then you and divdes the other side (500) to make 10, again. so you have left over 10=X. Thats just a better looking way to do it.