For this case we have:
x: Number of candies that Sydney has
The equation to find the amount of Sydney sweets is given by:

We want to clear x:



So, Sydney has 10 candies
Answer:
Sydney has 10 candies
No, Perry is not using the correct ratio, because 15:17 is not equivalent to 5:7
15 divided by 3 is indeed 5 but divided 17 by 3 does not give you 7, nor a whole number.
I hope this helps.