Subject: mathqa
Question: john purchased some shirts and trousers for $ 800 . he paid $ 400 less for the shirts than he did for the trousers . if he bought 5 shirts and the cost of a shirt is $ 20 less than that of a trouser , how many trousers did he buy ?
Option: 
A: 7
B: 4
C: 10
D: 6
Answer: C