If a hemisphere is cut out from one face, the surface area of remaining solid = total surface area of cube - base area of hemisphere + lateral surface area of hemisphere

Let the side of cube = a
radius of hemisphere = a/2

TSA of cube = 6a²
base area of hemisphere = π(a/2)² = πa²/4
LSA of hemisphere = 2π(a/2)² = πa²/2

So surface area of remaining solid = 6a² - πa²/4 + πa²/2
= 6a² + πa²/4
= (24a² + πa²) /4

 \frac{24+ \pi}{4}\ a^2

Surface area is   \frac{24+ \pi}{4}\ a^2
A tank can be filled by one tap in 20 minutes and buy another in 25 minutes are kept open for 5 minutes and then the second is turned off in how many more minutes is completely filled