Which pair of numbers is relatively prime?

a. 72 And 115

b. 120 And 835

c. 201 And 735

d. 303 And 1041

Relatively prime means having no factors in common.

I will start by checking if they have the factor "3" in common: it's easy to check: the sum of its digit needs to be divisible by 3:

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

the answer is a)