Subject: mathqa
Question: among the two clocks , clock a gains 20 seconds per minute . if clock a and b are set at 2 0 ' clock , when clock a is at 7 : 20 , what does clock b show ?
Option: 
A: 5 : 30
B: 6 : 20
C: 6 : 00
D: 3 : 30
Answer: C