Solution: (1) It is known that the crab transported from A crab breeding base to B city is (20-x) tons.

The crab transported from crab breeding base B to city A is (15-x) tons.

So the answer is: 20-x,15-x.

(2) From (1), the number of crabs transported from crab breeding base B to city B is 30-( 15-x)= 15+x,

So the algebraic expression containing X indicates that the total cost of transporting crabs this time is:

15x+ 12(20-x)+ 10( 15-x)+9( 15+x)，

Namely: 2x+525 ..

Ask again if you don't understand. I wish you progress in your study and go up a storey still higher! (*^__^*)