The first (by addition)

For example, a turntable has 12 equal parts, one red, three green, two yellow, and the rest are colorless. Q: What is the probability of going to the colored area?

Answer:1|12+3 |12 |12 = 6 |12 =1| 2.

In fact, these probabilities are essentially irrelevant, so addition is used.

The second (by multiplication)

Example: In a box, there are two yellow balls and three red balls. After mixing evenly, what is the probability that two (note that two) are randomly drawn and both are yellow balls?

2|5 ? 1|4 = 1| 10

In this case, multiplication is necessary because the front and back parts are related. Because you touched two balls, there are two probabilities. When I touched it for the first time, there were two yellow balls in five, so it was two fifths; On the second touch, there are only four balls left, among which there is only one yellow ball, so the probability is one in four. Add two and multiply to get one tenth.

?

A: 2 | 5? 1|4= 1| 10