Question

A basketball team played 15 games and won 80% of them. If the team expects to play 30 games in all, how many more games must it win to finish the season with a 90% winning percentage?

Answer 1

$\frac{(15 * 0.8) + x}{30} = 0.9$
$12+x=27$
$x=15$

They have alrady won 12 games (80% of 15) and need to win 15 more. 

Check:
$\frac{15+12}{30}=\frac{27}{30} = 90\%$