If x and y vary directly, that means that their ratio is always the same. In other words, x/y = k, where k is a constant. To find the value of k, we can use the information that x = 3 when y = 15:
x/y = k
3/15 = k
k = 0.2
Now that we know the value of k, we can use it to find x when y = 25:
x/y = k
x/25 = 0.2
x = 5