Bob and Wendy planned to walk from their home to a

Bob and Wendy planned to walk from their home to a restaurant for dinner together. However, Bob was delayed at work, and Wendy left for the restaurant before Bob did. If the restaurant is 3 miles from their home and Bob left for the restaurant a half-hour after Wendy did, how long did Wendy have to wait for Bob at the restaurant?

(1) Wendy walked at a constant pace of 4 miles per hour
Time = Distance/Rate
Time = 3/4
It took her 3/4ths of an hour (45 minutes) to get to the restaurant.

Still, we know nothing about about the rate at which Bob traveled after Wendy left.
INSUFFICIENT

(2) Bob walked at a constant pace of 1 mile per hour faster than Wendy.

We don't know Wendy's rate so we don't know Bob's either.
INSUFFICIENT

1+2)

Wendy rate = 4 miles/hour
Bob rate = 4+1 miles/hour

Time = Distance/Rate
Time = 3/5
It took him 3/5ths of an hour (36 minutes) to get to the restaurant.

She takes 45 minutes to get to the restaurant. Bob leaves when she is 15 minutes away. It takes him 36 minutes to get there from the point when she is 15 minutes away. In 15 minutes she is at the restaurant and bob is 36-15=21 minutes away.

She waits for 21 minutes.
SUFFICIENT

(C)