The number of ordered pairs (m, n), m, n∈{1,2,…50} such that 6n + 9m is a multiple of 5 is

All the numbers of the form 6n will end with 6 and 9m will end with 9, if m is odd and will end with 1, if m is even. Therefore, 6n + 9m will end with 5 if n is any number and m is odd.Thus, number of ordered pairs = 50 × 25 = 1250.