20th term =F + (N - 1) x D
38 = 10 + (20 -1 ) x D
38 = 10 + 19D
38 - 10 =19D
28 = 19D
D =28/19 =1 9/19 - The common difference
Sum =[F + L] /2 x N
Sum =[10 + 38] / 2 x 20
Sum =480, OR
Sum =N/2 [2F + (N-1) x 1 9/19]
Sum =20/2[20 + (19 x 1 9/19)
Sum = 480
