Question 1000012
They both started out with x, the unknown amount of money.<P>
David ended up with twice the original amount, which is 2x<P>
Daniel had the original x plus $12 from his birthday.  So he ended up with x+12.<P>
Daniel and David ended up with equal amounts, so 2x = 12+x.<P>
Subtract x from both sides.  x=12.<P>
Check:2x = 12+x<P>
2*12 = 12 + 12 = 24.