SOLUTION: how you get the sum of all the whole natural numbers greater than 89 and less than 101

One way is just to add up the 11 numbers:

90+91+92+93+94+95+96+97+98+99+100 = 1045

The other way is to use the sum formula for an arithmetic sequence:

Sn = (a1 + an) 

where n = 11, 

S11 = (a1 + a11)

then use

a1 = 90, and a11 = 100

S11 = (90 + 100)

S11 = (190)

S11 = (190) = 1045.

Edwin