 Hi and welcome to the session. Let's work out the following question. The question says find the sum of all two digit or positive numbers. Let us see the solution to this question. First of all we see that two digit or positive numbers are 11, 13, 15, 17, 19 and so on till 99. Here we see that the first term we denote it by A is equal to 11. Common difference that we denote by D is equal to 13 minus 11 that is equal to 2 and the last term say L is equal to 99. Now we know that An that is the nth term is equal to A plus n minus 1 into D where A is the first term, D is the common difference and n is the number of terms. This implies that 99 is equal to 11 plus n minus 1 into 2. This implies 99 minus 11 is equal to 2 into n minus 1, this implies 88 is equal to 2 into n minus 1 and this further implies that n minus 1 is equal to 88 divided by 2 that is equal to 44. Therefore n will be equal to 44 plus 1 that is equal to 45. We also know that sum of n terms denoted by Sn is equal to n by 2 into the first term that is A plus the last term that is L that is equal to 45 divided by 2 into 11 plus 99 that is equal to 45 divided by 2 into 110 that is equal to 45 into 55 that is equal to 2475. So our answer to this question is that the sum of all two digit or positive numbers is 2475. So I hope that you understood the solution. Have a good day.