Question

Two numbers 'a' and 'b'are chosen at random
from the numbers 1,2,3,....30. The chance that
a? - b2 is divisible by '3' is


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Answer 1

Find the probability that a2−b2 is divisible by 3. Total number of ways of choosing two numbers out of 1,2,3.... 30 is (302)=435.So total cases are 435. But i could not count the favourable number of cases and hence the probability.