Data Sufficiency

Don't have an OA for this one. However, I get the answer C

Detailed explanations would be appreciated. Many thanks in advance.

Half Distance = 60 yds
Jen's speed for 1st half = 4 yd/sec
Jen's TIME for 1st half = 60/4 = 15 secs
Jen's speed for 2nd half = 5 yd/sec
Jen's TIME for 2nd half = 60/5 = 12 secs
Jen's total time = 27 secs to cover 120 yds

St1: in 18 secs Jen covers (15secs * 4yd/sec) + (3sec * 5 yd/sec) = 75 yds
in 18 secs Linda covered 75 - 21 = 54 yds
To find avg speed for Linda, we need to know the time in which she covered the remeining distance
Hence Insufficient

St2: Jen completed race in 15 sec + 12 sec = 27 sec
Jen beats Linda by 3 secs
So Linda finished race in 30 secs
Linda's avg speed = 120/30 = 4 yd/sec
Sufficient

Ans B

Great explanation srcc25anu