Hope this helps and pls tell me if you like my handwriting
Answer:
a) 
b) 
Step-by-step explanation:
From the question we are told that:
3 coin are tossed
Therefore
Sample space 
Generally the sets that correspond with the result of at most one tail is therefore given as follows

Therefore
The result is at most one tail when

Answer: 7
Work: So, the goal is to get z alone, so we are going to add 85 on both sides, cancelling on the side it's currently on. So, the new equation is going to be 25z = 175. Then, to get z even more lonely, we are going to divide 25 on both sides, and again cancelling it on the side it's currently on. So, the answer is z = 7. I hope this helps, and Happy Holidays! :)