Dagonet's Juggling Balls

You could approach this puzzle in two ways.

You could either write down every number from 1-30 (since we know that half an hour is 30 minutes, which is how long Sir Dagonet will be juggling for) and go through the list number by number, adding 2 balls at every multiple of 4 and subtracting 3 balls at every multiple of 7.

You could also calculate how many 4 minutes are in half an hour, which would tell you how many times Dagonet has to add 2 balls, and then calculate how many 7 minutes are in half an hour, which is how many times Dagonet will drop 3 balls. You would then subtract the second number from the first.