I finally got this... a few additional pointers in case anyone else is as thick as me :)
Count CLUMPS of flowers, not individual flowers. I had one clump of white flowers but got confused trying to count individual ones. It's less confusing for the other colours because the clumps are more, well, clumpy.
I tried to overcomplicate the puzzle by thinking it was modulo arithmetic - it wasn't. It is just a transposition code. Here is a low tech way of doing it:
1. Write out the digits 0-9 on a bit of paper. Then take your first hint - for example 3=7.
2. On the row below, write "7" below the "3". Then continue the digits in order on the second row - wrap around when you get to the end, like this:
0 1 2 3 4 5 6 7 8 9 (row 1)
4 5 6 7 8 9 0 1 2 3 (row 2, using clue 3=7)
3. Then choose your flower numbers in the top row, and take the answer numbers from the second row. For instance, if your flowers are 1, 5, 3 you would get answers 5, 9, 7.
4. If that doesn't work then repeat the process for the other hints.
Finally, note that there are only 9 total possibilities for the solution - so if all else fails just do an exhaustive search. Slide row 2 above over a notch nine times and you will have all the possible solutions.