Discussion
There are two main entrances to a building with five floors. Each entrance leads to three lifts and each lift can stop at all the five floors. A person enters the building and reaches a floor. The number of possible ways that the person can reach the floor, is
Answer: D
There are 30 ways.
The person makes three independent choices in sequence: an entrance (2 ways), one of the three lifts from that entrance (3 ways) and a floor (5 ways). By the multiplication principle:
\[ 2 \times 3 \times 5 = 30 \]
15 forgets the choice of entrance, 10 forgets the choice of lift, and 25 and 50 do not correspond to the stages of the choice.
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