If x - y = xy then dy/dx is equal to? ​

Given :

php 65.50 = per hour
8 hours for 20 days = 160 hours overall


= php10,480 will be the amount he'll receive for compensation

Idont understand but my answer is 13th

The answer is 40, 41, 42 and 43

solution:
let x be the 1st number
x+1 be the second number
x+2 be the third number
x+3 be the fourth number
equation:
x + x+1 +x+2 + x +3 = 166
4x + 6 = 166
4x = 160
x=40