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CAT 2017 [slot 1] Question with solution 25

Question 25:
The number of solutions ( x, y, z) to the equation x - y - z = 25, where x, y, and z are positive integers such that $x\leq40,y\leq12$, and $z\leq12$ is
  1. 101
  2. 99
  3. 87
  4. 105
Option: 2
Explanation:

x - y - z = 25 and $x\leq40,y\leq12$, $z\leq12$
If x = 40 then y + z = 15. Now since both y and z are natural numbers less than 12, so y can range from 3 to 12 giving us a total of 10 solutions.Similarly, if x = 39, then y + z = 14. Now y can range from 2 to 12 giving us a total of 11 solutions.
If x = 38, then y + z = 13. Now y can range from 1 to 12 giving us a total of 12 solutions.
If x = 37 then y + z = 12 which will give 11 solutions.
Similarly on proceeding in the same manner the number of solutions will be 10, 9, 8, 7 and so on till 1.
Hence, required number of solutions will be (1 + 2 + 3 + 4 . . . . + 12) + 10 + 11
= 12×13/2 + 21
78 + 21 = 99


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