V of rectangular prism formula: L x W (some combine these two and say “area of the base”) x H.
They gave you the “area of the base” as 25/4 is in units squared. So you just need to multiply 25/4 x 8/5 = 200/20 which is 10 units cubed.
If decimals are easier 25/4 = 6.25 and 8/5 = 1.6. So 6.25 x 1.6 = 10 units cubed.