24. After playing 500 games, my success rate at Spider Solitaire is $49 \%$. Assuming that $\mathrm{I}$ win every game from now on, how many extra games do I need to play in order that my success rate increases to $50 \%$ ?
A 1
B 2
C 5
D 10
E 50
Solution: D
Since I have won $49 \%$ of my first 500 games, so far I have won $\frac{49}{100} \times 500=49 \times 5=245$ games. So I have lost $500-245=255$ games. I need now to win enough games so that I have won as many as I have lost. So, assuming I win every game from now on, I need to win $255-245=10$ more games.