John borrowed $1200 from his credit union at 18% simple interest. he agrees to repay the loan by making
30 equal monthly payments.
11. how much is the finance charge? (remember interest rate and time must have matching units.)

1200 (1+.8 * 30/12) = 1740.00
1740.00 - 1200  = $540

To find a GCF, find the prime numbers that multiply to equal the number you want. 

84 = 2 * 42 = 2 * 2 * 21 = 2 * 2 * 3 * 7

96 = 2 * 48 = 2 * 2 * 24 = 2 * 2 * 2 * 12 = 2 * 2 * 2 * 2 * 6 = 2 * 2 * 2 * 2 * 2 * 3 

Then you find the factors that each number (84 and 96) have in common:

2 and 2     2

2 and 2     2

3 and 3     3

So you multiply the respective common factors together:

2 * 2 * 3 = 12

And 12 is your GCF.