`7^m+7^n` divisible by 5
`7^n[7^(m-n)+1]`
`7^(m-n)+1` should be divisible by 5
Power of 7 has last digits revolving in 7,9,3,1.
`7^(m-n)` should have last digit as 9.
`7^(m-n)=7^(4p+2)`
`m-n=4p+2`
`4p+2<=100`<br> `p<=24.5`<br> favorable=25
total cases=100
`P=25/100=1/4`.