Final answer:
To find y when x = 39, first calculate the constant of proportionality k by dividing y by x for the given values, then use this constant and the new x value to calculate y.
Step-by-step explanation:
In mathematics, when y is directly proportional to x, the relationship can be represented by the equation y = kx, where k is the constant of proportionality. Given that y = 17.5 when x = 21, we can find k by rearranging the equation to k = y/x, leading to k = 17.5/21. Once we find k, we can determine y when x = 39 using the equation y = k * 39.
Step 1: Find the constant of proportionality, k.
k = 17.5 / 21
Step 2: Calculate y for x = 39.
y = k * 39
Substitute the value of k obtained in step 1 into this equation to find y.