Noting that 70% is the same as $\frac{70}{100}=\frac{7}{10}$, and that, when x is the amount of wins in the last 40 games, the fraction of games won is $\frac{40+x}{50+40}=\frac{40+x}{90}$, all we have to do is set them equal: $$\frac{40+x}{90}=\frac{7}{10}$$ $$40+x=63$$ $$x=\boxed{\text{(B)}\ 23}$$