What would be the probability that all reach safe?
$(\frac{9}{10})^{5}$ = 0.59049
Intuitively speaking, the probability of getting a head is $\frac{1}{2}$ then how many attempts would get you a head (provided once tail has come it wont appear again)?
$(\frac{1}{2})^{-1}$, right?
Here also, it would be $\frac{1}{0.59049}$ = 1.69350... ie roughly 1.7