Last year, Billy was 4 times as old as his son, Pierre. This year, their total age is 77 years old. How old will Pierre be when Billy is 2 times as old as him?
| |
Billy |
Pierre |
Difference |
Total |
| Before |
4x1 =
4 u |
1x1 =
1 u |
3x1 =
3 u |
|
| Change 1 |
+ 1 |
+ 1 |
|
|
| After 1 |
4 u + 1 |
1 u + 1 |
|
77 |
| Change |
+ ? |
+ ? |
|
|
After 2 |
2x3 =
6 u |
1x3 =
3 u |
1x3 =
3 u |
|
The difference in Ben's age and his son's age always remains unchanged. Make the difference in age the same. LCM of 3 and 1 is 3.
Total age of Billy and his son this year
= 4 u + 1 + 1 u + 1
= 5 u + 2
5 u + 2 = 77
5 u = 77 - 2
5 u = 75
1 u = 75 ÷ 5 = 15
Billy's age when Billy is 2 times as old as him
= 3 u
= 3 x 15
= 45
Answer(s): 45
