Explanation
Set up a table comparing the situation five years ago to the situation today to keep things organized.
Five years ago | Today | |
Mike | 10-5=5 | 10 |
Amy | 3x5=15 | 15+5=20 |
Mike is 10 years old now, so he must have been 5 years old five years ago.
Amy was 3 times as old as Mike five years ago, so Amy must have been 15 five years ago.
Add 5 years to Amy’s age five years ago to calculate her age now. Amy is 15 + 5 = 20 years old now.