The number of ordered pairs (m,n) m,n€{1,2,3,...,100} such that 7^m +7ⁿ is divisible by 5 is ___

to divisible by 5 unit digit must be either zero or 5 here exponent of 7 can be
(7,9,3,1) at unit digit and to make sum 0 or 5 it should be
(7+3) or (9+1)
to take 7+3
(7¹+7³) and (7⁵+7³)...................................(7⁹⁹+7³)

(7¹+7⁷)...........................................................(7⁹⁹+7⁷)
in this way you can apporach it