To solve this problem, create two equations with x and y. The pieces of information that we know are that the first child is 3 times older than the second child. Therefore if we assign child #1 the variable of x and child #2 the variable of y, we could get the following equation:
3x = y
Now, we also know that together their ages equal 50 months. Therefore:
x + y = 50 (months)
So, using these two equations you can use either substitution or elimination to find the values of x and y. You would first need to rearrange the first equation to 3x - y = 0 (just moving the y to the other side).
Then, you could add the equations together as followed:
3x - y = 0
x + y = 50
___________
4x = 50
x = 12.5
And then fill that value in to one of the equations to find y.
3x = y
3(12.5) = y
y = 37.5
So, one child is 12.5 months old and the other is 37.5 months old!