Here is a homework question for my 11 year nephew
" A father is 4 times as old as his son. In 6 years time he will be only 3 times as old. How old is each of them.

I don't know if there is an easy way to find out the answer, but I got it through trial and error.

If the son is 12, he's dad will be 48.
In 6 years, he will be 18 and his dad will be 54.

48 / 4 = 12
54 / 3 = 18

Let S be son's age now. Then in 6 years he will
be S + 6 years old
Let F be father's age now. Then in 6 years he will be
F + 6 years old

Now the father is 4 times as old as his son

F = 4S

In 6 years the father is 3 times as old as his son

F + 6 = 3(S + 6)

We solve this system of equations for F and S

Substitute the first equation into the second

4S + 6 = 3(S + 6)

4S + 6 = 3S + 18

S = 12

So the son is 12 years old and
the father is 48 years old