### Brain Teasers

# Mimi's Mathematical Moment

Fresh and invigorated after completing her algebra homework, Mimi appeared at the living room door. She held a piece of paper covered in scribbled numbers, 'm's, 'd's, and equations.

"Dad!" she announced. "Five years from now you'll be only double my age!"

"Amazing!" Dad almost looked up from the TV, but knew she hadn't finished.

"But five years ago, you were triple my age then!"

"I suppose in another ten years we'll be the same age," Dad grunted, changing channels.

How old is Mimi?

"Dad!" she announced. "Five years from now you'll be only double my age!"

"Amazing!" Dad almost looked up from the TV, but knew she hadn't finished.

"But five years ago, you were triple my age then!"

"I suppose in another ten years we'll be the same age," Dad grunted, changing channels.

How old is Mimi?

### Hint

Don't pay any attention to Dad's rather mindless comment at the end! He'd had a tough day.### Answer

Mimi is 15 (and Dad is 35).This one is probably easiest to guess and check. There are lots of little clues that help narrow down the options to guess. You might like to write some of these in the comments section.

e.g. Dad's age must have been a multiple of 6 five years ago. His age was triple Mimi's and ten years later it would be a multiple of two. So it was a multiple of both 3 and 2.

Or you could use algebra:

Let m and d represent Mimi's and Dad's ages now.

1. d-5 = 3x(m-5)

2. d+5 = 2x(m+5)

d-5 = 3m-15 ... *1*

d+5 = 2m+10 ... *2*

-10 = m-25 ... subtracting *2* from *1*

So m=15

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