If it happens, the probability is: 0.7 times 0.3, because the total number of * * * is 10 people, which can be counted separately. If there are 9 students, then 1 must be a teacher, so multiplying by 0.3 is the probability that a teacher will appear. 0.7 to the 9th power means 1-9. Not very clear, just understand it yourself!

If it is at least, we will discuss it in different situations. If it is nine, it is the answer above. If it is 10, it is 0.7 to the power of 10 for the same reason, so the final answer is: 0.7 to the ninth power times 0.3+0.7 to the power of 10.

There is something similar in the math book of senior two, please read it yourself!