In the given series, there is sum of multiple of corresponding terms of two A.P.’s. The two A.P.’s are
(i) 2 + 4 + 6 + 8 +....... to n terms and
(ii) 4 + 6 + 8 + ........ to n terms.
Now, the nth term of sum is
an = (nth term of the sequence formed by first A.P.) × (nth term of the sequence formed by second A.P.)
= (2n) × (2n + 2)
Put n = 20,
a20 = (2 × 20) × (2 × 20 + 2) = 40 × 42 = 1680.