Answer
The researcher needs to sample another 12 teenagers.
Work Step by Step
$p=0.90=90$%
We must have:
$np(1-p)\geq10$
$n0.90(1-0.10)\geq10$
$n\geq\frac{10}{0.90\times0.10}=111.111$
$n\geq112$ (The number of teenagers must be an integer)
But, the researcher sampled 50 teenagers. So, the researcher needs to sample another: $112-100=12$ teenagers.
