Basically 2+4+6+8+10+12+..............+n-1= 79*80 2(1+2+3+4+5+6+.......)=79*80 1+2+3+4+5+................= 79*40 Use Sum=n/2(2a + (n-1)d) , where a= 1st term , d= difference, n=number of terms n/2(2*1 + (n-1)) = 79*40 Rearranging, n^2 + n - 79*80=0 From the above equation, without doing any calculat...