First case (difference is 1)

numbers that can be formed are

(1,2) (2,3) (3,4)...........(99,100) total ways=99

Second case(difference=2)

numbers are

(1,3) (2,4) (3,5) (4,6)........(98,100) total ways=98

Third case(difference 3)

(1,4) (2,5) (3,6).........(97,100) total ways = 97

You can see that the number of ways is decreasing by1,so if we are asked to go upto 10 the number of ways would be

99+98+97+96......10terms

or

90+91+92.........99

By sum of AP