The least number of complete years in which a sum of money put out at 20% compound interest will be more than doubled is:

  $P1+\frac{20}{100}n>2P\iff \frac{6}{5}n>2$

  $\frac{6}{5}\times \frac{6}{5}\times \frac{6}{5}\times \frac{6}{5}>2$

so, answer is 4 years