Question:
Number of solutions of the equation 2x + y = 40 where both x and y are positive integers and x <=y ?
Solution:
2x + y = 40
x <= y
y = x + k; where k >=0
3x + k = 40
x = ( 40 - k ) / 3
x is a positive integer so takes value from 1 to 40 all multiples of 3
13 values.