Question: Can you give me some step-by-step instructions for this math problem: Four books are arranged on a shelf. The first book is 31 mm thick while the second book is 50 mm thick. The third book is 5 mm less thick than the second book, and the fourth book is twice as thick as the first book. What is the total thickness of the four books?
Answer: 
The third book is 50 - 5 = 45 mm thick.
The fourth book is 31 x 2 = 62 mm thick.
The total thickness of the four books is 31 + 50 + 45 + 62 = 188 mm.