Answer :
I've had a problem similar to this before. He should start out with 0 flowers. He would have 2 flowers at the first one, so he would put one on the first grave then have one for the second. When he gets to the second, he puts down one flower and the last flower on the 3rd grave.
he will start out with no flowers because then it will double at each river so 2x3=6
6/3=2 per grave
6/3=2 per grave