Question 763535
Frank is 15 years younger then John. 
<pre>
F = J - 15
</pre>
In 5 years John will be twice as old as Frank.
<pre>
J + 5 = 2(F + 5)

That gives us this system of equations:

F = J - 15
J + 5 = 2(F + 5)

We solve that and get

F = 10, J = 25
</pre>
How old will Frank be in four years?
<pre>
What is F + 4 ?

F + 4 = 10 + 4 = 14.

Edwin</pre>