Answer
The researcher needs to sample another 13 adult Americans.
Work Step by Step
$p=0.20=20$%
We must have:
$np(1-p)\geq10$
$n0.20(1-0.80)\geq10$
$n\geq\frac{10}{0.20\times0.80}=62.5$
$n\geq63$ (The number of adult Americans must be an integer)
But, the researcher sampled 50 adult Americans. So, the researcher needs to sample another: $63-50=13$ adult Americans.
