Intriguing Math Question

Today, in math class an intriguing math question. No matter how hard we tried, we couldn't solve it using a proportion. See, we have been working on doing math equations using proportions, and we haven't found a problem we couldn't solve - until now. The question is as follows:

"Trish's resting heart rate is 50 beats per minute. For every minute she excercises, her heart rate increases 5 beats per minute, How long will it take her to reach a heart rate of 120 beats per minute?"

A. 5 minutes
B. 14 minutes (Answer)
C. 34 minutes
D. 70 minutes

We were able to figure the answer was B, 14 minutes. We figured it out by doing it in our head, but
50//120 1//X
just doesn't add up at all. Do you have the answer? Remember, you have to use a proportion.

Thank you for your help! I'm counting on you!