From the first 100 natural numbers, two numbers first a and then b are selected randomly without replacement. If the probability that a - b 10 is mn, (m, n) = 1, then m + n is equal to .
Answer
311
Solution
Total ordered selections = 100 99 = 9900
For a - b = d (10 d 99): b = 1, 2, , 100 - d → (100 - d) pairs
Favourable = d=1099(100 - d) = 90 + 89 + + 1 = 90 912 = 4095
P = 40959900 = 91220 (dividing by 45)
m = 91, n = 220
m + n = 311
Answer: 311