Which pair of numbers is relatively prime? a. 18 and 105 b. 35 and 110 c. 70 and 79 d. 94 and 98

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 1157+2=91+1+5=7not true that both are divisible by 3b. 120 and 8351+2=38+3+5=16not true that both are divisible by 3but, they're both divisible by 5! (0 or 5 at the end)not relatively prime  c. 201 and 735 2+1=37+3+5=15 both are divisible by 3! d. 303 and 1041 3+3=61+4+1=6both divisible by 3 therefore the answer is a.