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

As the last digit of 6m,m∈N is 6,6m+9n will be divisible by 5 if the unit’s digit of 9n is 4 or 9. This is possible when n is odd.∴ required number of ordered pairs = 50 x 25 =1250.