Answer:
6
Step-by-step explanation:
Number of water bottles bought by Sylvia = 24
Number of water bottles sold by Sylvia = 
Number of water bottles left with Sylvia = 
Number of water bottles bought by Sam = 36
Number of water bottles sold by Sam = Three times as many as bottles as Sylvia sold = 
Number of water bottles left with Sam = 
It is given that the number of water bottles left with Sylvia and Sam are equal.
i.e.

So, number of water bottles sold by Sylvia = <em>6</em>