How many numbers between 1-100 are equal to five times an odd number?

There are 10 numbers between 1-100 which are equal to 5 times an odd number.

Explanation:

The odd numbers between 1 and 100 are 1,3,5,7…..

When these when these odd numbers are multiplied by 5 we get the following result.

These products form an AP 5,15,25,…..,95

common difference d=10

first term  

last term  

we have to calculate the number of terms n

The expression for last term of an AP is