Thanks sir,but i would appreciate if sir can explain to me why did sir put 10a for every sum of odd and even no.thanks

For all the odd (1st, 3rd, 5th....19th term) and even (2nd, 4th, 6th......20th term) terms, there are a total of 10 terms each. Using the formula a + (n-1)d, where a is the first term and will always be a constant. And when you add the terms, you will actually have 10 of a terms so this will correspond to 10a. Hope you understand

Oh I see.., anyhow thanks sir^^!!