Question

Martha deposits $500 in an account that pays 6% simple interest, and $1000 in a second account which has a higher interest rate but is more risky. What interest rate must she get on the second account in order to earn at least $130 in interest for the year?

Answer 1

$1000 x r%= 130
Since we can't move r% back two places to change it to a decimal, we can move the decimal by on the $1000.

Why? Is .47 x 53 the same as 47 x .53? (yes) - Do you see that is does not matter whether the decimal is moved back two places on one or the other?

So
$1000 can become 10 by moving it back two places.
10r = 130

10r = 130
10 10 divide both sides by 10 and you get 13%

Check 1000 x .13 = $130