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Now the solution:
Let the ages be F, M and S in years (whole numebers)
Since Father's age is divisible by son's age, therefore
F = S X n1
Since Mohter's age is divisible by son's age, therefore
M = S X n2
where n1 and n2 are positive whole numbers
Ratio of Father and Mother's age is 5:4
=> F/M = 5/4
Substitue for F and M
(S X n1)/(S X n2) = 5/4
=> n1/n2 = 5/4
=> n1 = 5/4 X n2
Since the sum of their ages is 70, therefore
F + M + S = 70
=> S X n1 + S X n2 + S = 70
=> S X 5/4 X n2 + S X n2 + S = 70
=> S(5/4 n2 + n2 + 1) = 70
Multiply by four both sides of the equation
=> S (5 n2 + 4 n2 + 4) = 280
=> S (9 n2 + 4) = 280
Substute positive integers values to n2, compute the value of (9 n2 + 4) and verify wether it is a factor of 280?
n2 (9 n2 + 4) Factor of 280
1 13 No
2 22 No
3 31 No
4 40 Yes
Therefore, n2 = 4, n1 = 5/4 X 4 = 5: n1 = 5
F = n1 X S = 5 S
M = n2 X S = 4 S
Since
S (9 n2 + 4) = 280
S (36 + 4)= 280, therefore,
S = 280 / 40 = 7
F = 5 S = 5 X 7 = 35
M = 4 S = 4 X 7 = 28
Therefore their ages are
Father = 35 years
Mother = 28 years
Son = 7 years
Good luck