@minipanda see,
We can make pairs for P1 P2,
(1,8)(2,7)(3,6)(4,5)
At any moment maximum req for both the process is 8.
Hence we can give 7 to them and 1 will be backup resource and same logic with R3. we can give 5 to it and already we have 1 backup resource for completion
Hence total will be 7+5+1 = 13