Question

5 years from now a boy's age will be 2 time the square of what it was 5 years ago.how old is he now?
a)15/2 years c)7 years

b)3 years d) 8 years

Answer 1

age now: x
age in 5 years: x+5
age 5 years ago: x-5

so, write the words in math

x+5 = 2(x-5)^2

now just solve for x. Since you have a quadratic function, you will get two answers. Only one answer will make sense.

Answer 2

Now: X years old.
5 yrs. ago: x-5 yrs. old.
5 yrs. from now: x+5 years old.

x+5 = 2*(x-5)^2.
x+5 = 2(x^2-10x+25).
x+5 = 2x^2-20x+50,
-45 = 2x^2-21x,
2x^2-21x+45 = 0.
Use Quadratic Formula:
X = (21 +- sqrt(441-360))/4 = 30/4 = 15/2, and 3.
Use the larger number:
X = 15/2 = 7.5 Years old.