The equation of a straight line passing through points \(P(x_1, y_1)\) and \(Q(x_2, y_2)\) is: \(\frac{y-y_1}{x-x_1}=\frac{y_1-y_2}{x_1-x_2}\) (check here:
http://gmatclub.com/forum/math-coordina ... 87652.html).
For P(0, 30) and Q(50, 0): \(\frac{y-30}{x-0}=\frac{30-0}{0-50}\) --> \(3x + 5y = 150\).
If x is a multiple of 5, then y will be an integer. x ranges from 0 to 50, inclusive. There are total of 11 multiples of 5 in this range: 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50.
Answer: D.