So you have to solve for "donation" - so let's call that X. And the things we know are what the grandfather donated, as a function - meaning based upon - Theo's donation. We'll call that Y.
So Y is equal to 1/2 of $4 plus Theo's donation...... 0.5*(x+4).
So... 4.25 = 0.5(X+4), solving for X
4.25=0.5x + 2
-2 -2
2.25=0.5x - divide each side by 0.5 to get x alone.