Question

Dorothy is 7 years older than her brother William. If the product of their ages is 78, how old is Dorothy?

Answer 1

Answer:
Dorothy = 13 years old

Step by step answer

Dorothy: x+7
William: x

x * (x+7) = 78
x^2 + 7x = 78
x^2 + 7x -78 = 0
What adds to 7, multiply to 78?
-6, 13
y=(x-6)(x+13)

Find the zeros
x-6=0
x=6 ( william is 6 years old)

x+13=0
x=-13 (age can’t be negative)

Dorothy = x+7
6+7 = 13