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?
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Let's assume that x and y are the current ages of the older man and the younger man, now we can write:
⎩ ⎨ ⎧ x − 1 0 x + 1 0 = 3 ( y − 1 0 ) . . . . . ( 1 ) = 2 ( y + 1 0 ) . . . . . ( 2 )
Now, from ( 1 ) :
x − 1 0 x x = 3 ( y − 1 0 ) = 3 y − 3 0 + 1 0 = 3 y − 2 0 . . . . . ( 3 ) .
Substituting x = 3 y − 2 0 in ( 2 ) :
3 y − 2 0 + 1 0 3 y − 1 0 3 y − 2 y ⟹ y = 2 y + 2 0 = 2 y + 2 0 = 2 0 + 1 0 = 3 0 .
Hence the current age of the younger man is 30 years. We don't have to solve for x now.
If you don't want to solve for x , you can take the first equation and subtract the second equation. It's faster that way.
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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