AMO 2027 Grades 9-11 SET 5 - Self-study Question 1

A dog barks every $b$ minutes and a cat meows every $m$ minutes. Both $b,m$ are primes with $1\le b\le10$ and $10\le m\le20$. What is the greatest number of simultaneous calls in a 2-hour period? To make the time convention explicit, both call at time 0; count only calls with $0<t\le120$ minutes.

Choose one option.