5.

Find the sum of all two - digit natural numbers which are divisible by 3.

1665

Step-by-step explanation:

This is AP, where:

First term is: 12 = 3*4 Last term is: 99 = 3*33 Common difference is: 3 Number of terms is: 33 - 3 = 30 (because the 4th term is included) S=1/2*30 (12+99) = 1665

The sum of all two - digit natural numbers which are divisible by 3 is 1665