The Lees have 3 children. The oldest is twice as old as the youngest. The middle is 5 years older than the youngest. If the sum of the age is 57, how old is each child?

Oldest = 2 times Youngest - > O = 2*Y

Middle = Youngest + 5 - > M = Y+5

All of them together is 57 - > O + M + Y = 57

So you have these three equations:

(1) O = 2*Y

(2) M = Y+5

(3) O + M + Y = 57

Now you want to reduce the number of variables. You can change the second equation to be Y = M-5 and then plug in "M-5" wherever there is currently a Y:

(4) O = 2 * (M-5) = 2*M - 10

(5) O + M + (M-5) = 57

which becomes O + 2M = 62

Then you plug in the "O" equation (4) into (5) which gives you

(2M-10) + 2M = 62 which reduces to 4M = 72.

So now I know M is 18.

I can now plug that into my other equations:

(4) O = 2*18 - 10 which means O = 26.

Now I plug that into (1) from the top:

26 = 2*Y which becomes 13 = Y

So now I have O, Y, and M

Oldest is 26

Middle is 18

Youngest is 13

Reading the sentence again, you can see that this makes sense.