After the 3rd hour
8 lily pads
This is quite straight forward and require no deep mathematics.
Harry has 20 lily pads to eat and he eats 4 per hour. After the first hour, he eats 4 and he has 16 left. After second hour, he eats another 4 and he has 12 left. After third hour, he eats four and has eight left.
For Hennrick, he has 29 lily pads and he eats 7 per hour. After first hour, he has 22 left. After second hour, he eats another 7 and have 15 left. After 3rd hour, he eats another 7 and has just 8 left.
8 lily pads will be left at the same time