Common mistake is to consider the price for both the cars is the same.

for the car on which the dealer made the profit let us assume the cost was x.
Then,
1.25x = 40000
or, x = 32000
Therefore, the net gain is 8000.

for the car on which the dealer suffered loss. let us assume the cost was y.
0.80y = 40000
or, y = 50000
therefore, the net loss is 10000

Hence B>A