avg.

Since there are $20$ numbers, we let $x$ be the first $19$ numbers, then

$\dfrac{x+61}{20}=35$

$x+61=700$

$x=700-61=639$

Now we change $61$ to $16$ . We have

$\dfrac{x+16}{20} = \dfrac{639+16}{20}=\boxed{32.75}$

Let the sum of 20 numbers be x. => x/20 = 35 or x = 700 ---> (1).

Now he wrote 61 instead of 16.. So the new average will be :- (x-45)/20.

By putting the value of x from Eq.1 and solving we get :-

Average=(700-45)/20.

=>Average=(655)/20.

=>Average =32.75 .