Finding the Third Proportional to 639.91 and 978.47

Step by Step Solution

1. Step 1: Write the proportion equation
   639.91 : 978.47 = 978.47 : x
2. Step 2: Convert to fraction form
   639.91/978.47 = 978.47/x
3. Step 3: Cross multiply
   639.91x = 978.47 × 978.47
4. Step 4: Simplify right side
   639.91x = 957,401.09
5. Step 5: Solve for x
   x = 957,401.09/639.91
6. Step 6: Final answer
   x = 1,496.14