This problem can be solved by the solution of "chicken and rabbit in the same cage", assuming that all of them are sold at 40%, you can earn more. What is extra is to calculate 20% as 40%, so that the cost price of A can be calculated, and then the cost price of B can be calculated.

Formula: (2700*40%-840)/(40%-20%)

=( 1080-840)/20%

=240/20%

= 1200 (yuan)

2700- 1200= 1500 (yuan)

A: The cost price of A is 1200 yuan, and the cost price of B is 1500 yuan.