Final answer:
Sonia's research paper was 34 pages long.
Step-by-step explanation:
To determine the number of pages in Sonia's research paper, we can set up an equation using the given information. Let's denote the number of additional pages as x. We know that the total cost to Sonia is $100.
According to the pricing scheme, the first 3 pages cost $7, and each additional page costs $3. So, the equation becomes:
7 + 3x = 100
To solve this equation, we isolate the variable x by subtracting 7 from both sides:
3x = 93
Finally, divide both sides by 3 to solve for x:
x = 31
Therefore, Sonia's research paper is 31 additional pages long, making the total number of pages 31 + 3 = 34 pages.