Answer:
Check the difference between each two / each pair if buyer and seller.
(note that the surplus could be split between them, making it effectively a win-win-scenario. but it could also be extremely good for one of them, yet just at the limit for the other one)
a) $11
b) $8
c) $6
d) add every max. buying price up ($64) and do the same with all the minimum selling prices ($33)
the difference between these two is your answer: $31
