Explanation:
the volume of the tank is
80×50×45 = 180,000 cm³
3/4 of that volume is
180000 × 3/4 = 135,000 cm³
so, we want to know how long it takes to fill 135,000 cm³.
the water intake flow is 9 l / minute.
now we need to know that a cube of 10 cm side length contains exactly 1 liter of water.
such a cube has the volume
10×10×10 = 10³ = 1000 cm³
so, 1000 cm³ = 1 liter.
and we need to fill 135,000 cm³.
how many times fit 1000 cm³ into 135,000 cm³ ?
135000/1000 = 135 times.
and that means it would take 135 minutes to fill it, if the flow was 1 l / minute.
but it is faster (9 times faster) : 9 l / minute
so, how many minutes ?
135 / 9 = 15 minutes
it takes 15 minutes to fill 3/4 of the tank.