What is the maximum value of the objective function under then given constraints?

If your draw the the three lines using the same grid, you will see that the three inequality share a region of a right triangle with the vertex (2,5), (2,3), and (6,3)
(2≤x≤6, and 3≤y≤5)
when x=6, y=3, C=2*6+5*3=27
when x=2, y=5, C=2*2+5*5=29
when x=2, y=3, C=2*2+5*3=19
the maximum value for C is 29

Time. Water

0.5. 0.8

1. 1.6

3. 4.8

25. 40