The faster of two runners on a circular race course can complete a lap in 30 seconds. The slower runner can complete a lap in 40 seconds. The runners start from the same spot at the same time. How long will it take the faster runner to gain a full lap on the slower runner?
I can easily do this problem in my head and come up with 120 seconds. I just can't figure out how to write it down and solve it using the distance = rate x time formula. I'm embarrassed to ask but can someone help?