Question 860348
let x be your mother's age
40 = x + 4
let y be your age
x = 3y
solve first equation for x
x = 36
substitute for x in second equation
36 = 3y
3y = 36
y = 12
you are 12 years old
your sister is .75 * 12 = 9 years old
let z be your grandmother's age
.15 * z = 9
z = 9 / .15 = 60
your grandmother is 60 years old