Example 3. The germination rate of seeds of this plant is $90 \%$. Find the probability that out of four sown seeds, the following will germinate: a). three; b) no less than three.
Solution. The required probabilities are found using the Bernoulli formula.

In the first case $n=4, k=3, p=0.9, q=1-p=0.1$, therefore

$$
P_{4}(3)=C_{4}^{3} p^{3} q=4 \cdot 0.729 \cdot 0.1=0.2916
$$

In the second case, event $A$ consists of either three or four seeds sprouting out of four. By the addition theorem of probabilities, $P(A)=P_{4}(3)+P_{4}(4)$. Since $P_{4}(4)=C_{4}^{4} p^{4} q^{0}=(0.9)^{4}=0.6561$, then

$$
P(A)=0.2916+0.6561=0.9477
$$