7. the last one 
I = P R T 
I = 15.75   P = 500  R = unknown  T = 6
15.75 = 500 (r)  (6) 
divide the T and I
15.75 ÷ 6 = 500 (r)  (6) ÷6
2.62 = 500 (r)  *get rid of the six*
divide the P with new answer  
2.62 ÷ 500 = 500 ÷ 500 *get rid of 500*
0.00524 = r
move decimal to make it in to a percentage 
5.24% = R