Find the number of terms of the AP 18, 15, 12, ... so that their sum is 45. Explain the double answer.

Find the sum of all natural numbers between 100 and 500 which are divisible by 7.

Find the number of terms of the AP 64, 60, 56, ... so that their sum is 544. Explain the double answer.

Find the number of terms of the AP 63, 60, 57, ... so that their sum is 693. Explain the double answer.

How many terms of the AP 21, 18, 15, ... must be added to get the sum 0?