Who talks like that?

Algebra Level 2

Two grown men are having a conversation. The older man says,

 "10 years ago, I was triple your age.     
 10 years from now, I will be double your age."

What is the current age of the younger man?


The answer is 30.

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2 solutions

Francesco Iacca
Mar 31, 2017

We can call the current age of the older man X, the age of the younger Y. Then we can solve this system

X-10=3(Y-10)

X+10=2(Y+10)

Solving this system we find that X=70, Y=30. So the answer is 30

Munem Shahriar
Mar 18, 2018

Let's assume that x x and y y are the current ages of the older man and the younger man, now we can write:

{ x 10 = 3 ( y 10 ) . . . . . ( 1 ) x + 10 = 2 ( y + 10 ) . . . . . ( 2 ) \large \begin{cases} x -10 & = 3(y-10) ~~~~ .....(1) \\ x +10 & = 2(y+10) ~~~~ .....(2) \\ \end{cases}

Now, from ( 1 ) : (1):

x 10 = 3 ( y 10 ) x = 3 y 30 + 10 x = 3 y 20 . . . . . ( 3 ) . \begin{aligned} x-10 & = 3(y - 10) \\ x & = 3y - 30 +10 \\ x & = 3y - 20 ~~~~ .....(3).\\ \end{aligned}

Substituting x = 3 y 20 x = 3y - 20 in ( 2 ) : (2):

3 y 20 + 10 = 2 y + 20 3 y 10 = 2 y + 20 3 y 2 y = 20 + 10 y = 30 . \begin{aligned} 3y - 20 +10 &= 2y + 20 \\ 3y - 10 & = 2y + 20 \\ 3y - 2y & = 20 + 10 \\ \implies y & = \boxed{30}.\\ \end{aligned}

Hence the current age of the younger man is 30 years. We don't have to solve for x x now.

If you don't want to solve for x x , you can take the first equation and subtract the second equation. It's faster that way.

Pi Han Goh - 3 years, 2 months ago

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