I don't know if i'm wrong, but this might help. Sorry of it seems complicated, I tried to explain a little just in case.
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Question 1.)
There were 24 plants planted in total, with 1 flower planted by a girl per 3 trees planted by the boys. Just find 1/4 of 24, then you can probably figure out the rest.
Question 2.)
The parts can be made into a fraction over 5 to make things a little easier if you want. Find 1/5 of 500 for the amount of concentrate, and 4/5 for the amount of water. Split the 100ml lost between both numbers. add the 100ml lost back onto the concentrate amount. Find the final percentage of the concentrate.