In this Problem you can use the Cross Multiplication.
If in 15 seconds the nurse counted 18 beats, so in 60 seconds she will count x beats.
15 seconds = 18 beats
60 seconds = x beats
You use the cross multiplication and multiply 60 by 18 and 15 by x, so you get:
15x = 60 * 18
15x = 1080
Then, you divide 15x and 1080 by 15 each, so you get:
x = 72
So, the Nurse counted 72 beats in 60 seconds
Yourwelcome :)
