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.