Question

Why do we subtract 1 to find out whole numbers between two numbers?

Answer 1

Because to leave both end numbers

example:
find how many numbers between 0 and 100
i) excluding both ends
ii) including both ends
iii) include one end

solution:
i) The question say to find how many numbers between 0 and 100 and leave both end numbers i.e leave 0 and 100
so, no of numbers between 0 and 100 = 100 - 0 - 1 = 100 - 1 = 99

so, there are 99 numbers between 0 and 100

ii)The question say to find how many numbers between 0 and 100 and including both end numbers i.e count 0 and 100
so, no of numbers between 0 and 100 = 100 - 0 + 1 = 100 + 1 = 101

so, there are 101 numbers between 0 and 100 including both ends

iii)The question say to find how many numbers between 0 and 100 and leave one end number i.e leave 0 or 100
so, no of numbers between 0 and 100 = 100 - 0 = 100 = 100

so, there are 100 numbers between 0 and 100 include one end number
Note :
i) if you want to leave both end numbers , use -1
ii) if you want to include both end numbers , use +1
iii)if you want to leave one end number , leave as it is