CPM Homework Help : CCA2 Problem 12-107

Hospital records show that $10\%$ of all the cases of a certain disease are fatal. If five patients suffer from this disease, find the probability that:

All will recover.
Hint (a):
If $10\%$ die, what percentage recover?
More Help (a):
Another way to think of this is that none of the patients die.
Step (a):
$_5C_0(0.9)^5(0.1)^0$

At least three will die.
Hint (b):
Add up the probabilities that $3$ die, $4$ die, and $5$ die.
Step (b):
The probability that $3$ die is
$_5C_3(0.9)^2(0.1)^3.$