Answer
Part 1: 72 times/minute
Part 2: 150 times/minute
Part 3: 234 times
Step-by-step explanation
Part 1
18 times in 15 seconds when at rest.
In 1 second = 18/15
= 1.2 times/second
In 1 minute = 1.2×60
= 72 times/minute
Part 2
25 times in 10 seconds while running.
In 1 second = 25/10
= 2.5 times/second
In 1 minute = 2.5×60
= 150 times/minute
Part 3
In 3 minutes while at rest = 72×3 = 216
In 3 minutes while running = 150 × 3 = 450
How many more times does your friend's heart beat in 3 minutes while running than while at rest?
Answer = 450 - 216
= 234 times