First of all, we can use unit conversion to calculate how many times we need to pump every hour. The number of times that 20 ml of saline needs to be pumped per hour is 20 ÷ 2.5 = 8 times.

Then, we can calculate how long it takes to finish each pump. Pump 2.5 ml each time, so the time required for each pump is 2.5 ÷ 20 × 60 = 7.5 minutes.

Finally, we can calculate how long it will take to pump 20 ml of salt water. Because it needs to be pumped 8 times per hour, the time required for each pump is 7.5 minutes, so the total time required for pumping 20 ml of saline water is 8 × 7.5 = 60 minutes, that is, 1 hour.

Therefore, it takes 1 hour to pump 20 ml of saline.