The individual should work 28 hours to earn $630 at a rate of $22.50 per hour.
Direct variation formula is y = kx;
Where y directly varies on the changes of x with the constant ratio of k.
We need to first get the k or constant rate.
y = kx
900 = k (40)
k = 900 / 40
k = 22.50 this is the rate per hour
To check:
900 = 22.50 * 40
900 = 900
y = 22.50x
630 = 22.50x
x = 630 / 22.50
x = 28 hours.
To check:
630 = 22.50 * 28
630 = 630