Last digit of 74p+1=7 Last digit of 74p+2=9 Last digit of 74p+3=3 Last digit of 74p=1 If m is 4p+1, then n should be 4p+3. So that 7m+7n is divisible by 5 and vice-versa also. Similarly, if m is 74p+2 then n should be 74p and vice-versa to be divisible by 5 So, number of ordered pairs =4×(25C1×25C1)=2500