Find the total number of ways in which 20 balls can be put into 5 boxes so that first box contains just one ball.

Any one of the twenty balls can be put in the first box. Thus, there are twenty different ways for this.

Now, remaining 19 balls are to be put into the remaining 4 boxes. This can be done in $4^{19}$ ways because there are four choices for each ball.

$\therefore$ Required number of ways $=20 \times 4^{19}$