Question

Dorothy is 10 years older then her brother. If the product of their ages is 231, how old is dorothy?

Answer 1

Hi , Dorothy is 21 years old and her brother is 11 years old , because 21 times 11 equals to 231 , the product of their ages is 231 , in other words when you multiply those two ages you will get 231.

Answer 2

d * b = 231
d = b + 10

b(b + 10) = 231
b^2 + 10b = 231
b^2 + 10b - 231 = 0
(b - 11)(b + 21) = 0

b - 11 = 0
b = 11 <== brothers age

b + 21 = 0
b = -21.....extraneous solution...does not work

d = b + 10
d = 11 + 10
d = 21 <== dorothy's age

so dorothy is 21 and her brother is 11