From above, just to generalize "No of multiples in a given range = (b-a)/11 + 1" is also =(b-a) / size of interval between number +1. Interval here is 11. That is, if you look at the first few terms, they are 110, 121, 132. Interval between 121 and 110 is 11. Interval between 132 and 121 is 11, etc. It's multiples of 11 so each term has 11 between them.

But if this question was just asking for number of terms for all integers between 110 and 990, then it'd be [(990-110)/1] + 1 instead.

100 to 1000 = 901 terms
901/11 = 81 terms divisible by 11