Your question is 200-Page Book Scheduling. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Person A and person B both need to write a 200-page book. Person A writes 20 pages per day, and person B writes 30 pages per day. How much earlier will person B finish writing the book?