The key is to find the completion time t, the work efficiency of A is 1/t, and the work efficiency of B is 1/(5+t).

4( 1/t+ 1/(5+t))+(t-4)(5+t)= 1

Calculate t=20.

The cost of scheme 1 is 1.5*20=30.

The cost of the second scheme is 1. 1*25=27.5.

The cost of the third scheme is 2.6 * 4+16 *1.1= 28.

The title says it won't exceed the specified time, so the answer is plan 3.