$16 is what percent is 80$

16 = n% x 80
16 = .8n     (since you can't move the decimal 2 places back on n%, move the decimal point two places back on the number you are multiplying n% by.  
Look at this sample to see that it works:    .83 x 17 = 83 x.17

16   = .8 n
.8       .8     divide both sides by .8

Answer is 20%.    $16 is 20% of $80.

16 is what percent of 80
= 16 / 80
= 0.2

decimal to a percentage:
0.2 * 100 = 20%