5% of 11 is 0.55. What number times 0.55 = 11? Well if we divide 11 by 0.55, we see that the answer is 20. So, the population will double in 20 years. If the population is 22 in 20, it stands to reason that it will take another 20 years to double from 22 to 44. So, the population will reach 44 in 40 years.
Hope this helps.