What is the probability of two or more from n events occuring? A number of independent events, say $A$, $B$, $\ldots\,$, $E$, can happen with associated probabilities $P(A)$, $P(B)$, $\ldots$
For each event that happens I have to pay £10. The likelihood I have to pay at least £10 is straightforward to work out:
$$1 - \bigl(1-P(A)\bigr)\bigl(1-P(B)\bigr)\bigl(1-P(C)\bigr)\bigl(1-P(D)\bigr)\bigl(1-P(E)\bigr)$$
But what is the likelihood that I have to pay out at least £20? ie what is the likelihood of at least two events occuring? 
For 5 events I could list out all 32 possible outcomes (from none of the events happen through to all 5 events happen) and work out the probability of each outcome. However this approach is not feasible if I have 10s of events.
 A: We first give an answer to the question as put. Then we deal with $n$ events. In a sense, the general case turns out to be simpler than the $A,\dots,E$ case, since the structure is more evident. 
For simplicity let $\Pr(A)=a$, $\Pr(b)=b$, and so on. The probability that  at least $2$ of $A,\dots, E$ happen  is $1$ minus the probability that at most $1$ of our events happens. 
The probability of at most $1$ is the sum of two probabilities: (i) the probability of none and (ii) the probability of exactly $1$,
(a) You calculated this already: it is $(1-a)(1-b)(1-c)(1-d)(1-e)$.
(b) We get exactly $1$ if $A$ happens but the others don't, or if $B$ happens but the others don't, and so on. These events are pairwise disjoint, so the probability of exactly $1$ is
$$a(1-b)(1-c)(1-d)(1-e)+(1-a)(b)(1-c)(1-d)(1-e)+\cdots
$$
(the sum contains $5$ terms, of which we have shown the first two).
The general case: Let the independent events be $A_1,A_2,\dots,A_n$  Let $\Pr(A_i)=p_i$. Then the probability $A_i$ doesn't happen is $1-p_i$. It is convenient to let $1-p_i=q_i$. (This is a standard abbreviation.)
The probability that $0$ of the events happen is then given by the pleasant expression $q_1q_2q_3\cdots q_n$. Call this $P_0$.  
The probability exactly $1$ of the $A_i$ happens is the sum of $n$ terms, where the $i$-th term is $q_1q_2\cdots q_{i-1}p_iq_{i+1}\cdots q_n$. Call this sum $P_1$. 
Then the probability that $2$ or more of our events happen is $1-P_0-P_1$.    
Computational note: The calculation is not nearly as ugly as it seems. Assume that none of the $q_i$ is $0$. Then the probability of exactly $1$ can be rewritten as
$$\left(q_1q_2\cdots q_n\right)\left(\frac{p_1}{q_1}+\frac{p_2}{q_2}+\cdots+\frac{p_n}{q_n}          \right).$$
So the probability of at least $2$ is 
$$1-\left(q_1q_2\cdots q_n\right)\left(1+\frac{p_1}{q_1}+\frac{p_2}{q_2}+\cdots+\frac{p_n}{q_n}          \right).$$
