The sum of the age of andy and his parents is 72.In how many years time will the sum of their age be 90?

3 answers

(90-72)/2 = 9
a = Andy's present age

m = Mom's present age

d = Dad's present age

m + d + a = 72

After x years:

Mom will be m + x yrs old

Dad will be d + x yrs old

Andy will be a + x yrs old

m + x + d + x + a + x = 90

m + d + a + 3 x = 90

We know:

m + d + a = 72

So:

72 + 3 x = 90 Subtract 72 to both sides

72 + 3 x - 72 = 90 - 72

3 x = 18 Divide both sides by 3

x = 18 / 3 = 6

After 6 years the sum of their age be 90.
I neglected the fact that there were two parents...

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