Question

Assume that the number of sales per day of an app in the apple ios app store is normally distribbuted.

Answer 1

You need to know the distribution's mean and standard deviation.

$\mathbb P(X>100)=0.20\implies\mathbb P(X\le100)=0.80$

Transforming to the standard normal distribution, and letting $\sigma$ denote the standard deviation,

$\mathbb P(X\le100)=\mathbb P\left(\dfrac{X-86}\sigma\le\dfrac{100-86}\sigma\right)=0.80$

The z-score corresponding to this probability is approximately $z=0.8416$, so you have

$\dfrac{100-86}\sigma=0.8416\implies\sigma=16.64$