Which pair of numbers is relatively prime?
a. 72 and 115 b. 120 and 835 c. 201 and 735 d. 303 and 1041

a. 72 And 115
7+2=9
1+1+5=7
not true that both are divisible by 3
b. 120 And 835
1+2=3
8+3+5=16
not true that both are divisible by 3
but, they're both divisible by 5! (0 or 5 at the end)
not relatively prime
 c. 201 And 735 
2+1=3
7+3+5=15 
both are divisible by 3!
d. 303 And 1041 
3+3=6
1+4+1=6
both divisible by 3 
so the answer is a 

Where are the following equations?