Let's first work out the volume of the pool.

V = L x B x H

= 90 x 4 x 1

= 360 cubic metres

360 x 1000 = 360 000 litres.

If the pump can pump 100 litres in a minute, it can pump 6000 litres in an hour.

So the time taken to fill the pool in hours is: 360 000 / 6000 = 60 hours.