JAMB - Mathematics (1978 - No. 10)
Five years ago, a father was 3 times as old as his son, now their combined ages amount to 110years. thus, the present age of the father is
75 years
60 years
98 years
80 years
105 years
Vysvětlení
Let the present ages of father be x and son = y
five years ago, father = x - 5
son = y - 5
x - 5 = 3(y - 5)
x - 5 = 3y - 15
x - 3y = -15 + 5 = -10 ......(i)
x + y = 110......(ii)
eqn(ii) - eqn(i)
4y = 120
y = 30
sub for y = 30 in eqn(i)
x -10 = 3(30)
x -10 = 90
x = 80
five years ago, father = x - 5
son = y - 5
x - 5 = 3(y - 5)
x - 5 = 3y - 15
x - 3y = -15 + 5 = -10 ......(i)
x + y = 110......(ii)
eqn(ii) - eqn(i)
4y = 120
y = 30
sub for y = 30 in eqn(i)
x -10 = 3(30)
x -10 = 90
x = 80