At 9 a.m., Ryan had finished cutting $\frac{1}{2}$ of his lawn. At 10 a.m., he had finished cutting $\frac{7}{8}$ of his lawn. If Ryan cut his lawn at a constant rate, at what time did he finish?
(A) 10:15 a.m.
(B) 11:20 a.m.
(C) 10:20 a.m.
(D) 10:30 a.m.
(E) 11:40 a.m.
Between 9 a.m. and 10 a.m., Ryan cut $\frac{7}{8}-\frac{1}{2}=\frac{7}{8}-\frac{4}{8}=\frac{3}{8}$ of his lawn.

Ryan cut $\frac{3}{8}$ of his lawn in 1 hour (60 minutes), and so he cut $\frac{1}{8}$ of his lawn in $\frac{60}{3}$ minutes or 20 minutes.

At 10 a.m., Ryan had cut $\frac{7}{8}$ of his lawn, and thus had $1-\frac{7}{8}=\frac{1}{8}$ of his lawn left to cut. Since Ryan cuts $\frac{1}{8}$ of his lawn in 20 minutes, then he finished at 10:20 a.m.

ANSWER: (C)