Help me please

Question

two frog live together in a backyard pond. One frog croaks every 4 minutes, the other croaks every 2 and 1/2 minutes. If they both croak together at 11am, what is the first time after 1pm that they will croak together?

Answer 1

Divide time into half-minutes, so Frog 1 croaks every 8 half minutes and Frog 2 croaks every 5 half-minutes. Therefore the LCM of 5 and 8=40 half-minutes=20 minutes. So the first time they croak together after 11am is 11:20am. They continue to croak together every 20 minutes: 11:40am, midday, 12:20pm etc. Then we get to 1pm and 1:20pm, the first time after 1pm when they croak together.