Starting off, you already know he has 4 daughters and for every daughter there are 4 sons.
So, we take the 4 daughters and add 4 to each one:
1 daughter + 4 sons = 5 children
We complete that equation 4 times for each daughter, and we end up with 20 children as our answer.
Hope this helps you out a bit! Stay safe and stay healthy! :D
