It is given that Sylvia sells x number of bottles and Sam sells three times as many bottles as Sylvia sold to his co-workers.
So, number of bottles Sam sold is 3x.
It is also given that, number of remaining bottle Sylvia and Sam have same.
So,
24 - x = 36 - 3x
3x - x = 36 - 24
2x = 12
x = 6
Therefore, Sylvia sells 6 bottles.
Hence, this is the required solution.