Solution
Sum of no. = $$\frac{no.\ of\ term}{2}\times(first\ term+last\ term)$$
Avg = $$\frac{sum\ of\ no.}{no.\ of\ term}$$
= $$\frac{first\ term+Last\ term}{2}$$
First term which is multiplied by 6 after 20 is 24 and last no. Is 78
=$$\frac{24 +78}{2}$$
=51
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