How to solve these type of questions

Hello, how to implement and check answer for following question

You have three program statuses in your Email Send channel, in this order and with this amount of leads at each stage:
Sent (Step 10): 10 members /
Opened (Step 20): 10 members /
Clicked (Step 30 - Success): 15 members.
Based on this setup, the total number of people who opened this email is 10.
How many are the total members in the program?
10
20
30
35