Finding the Third Proportional to 639.75 and 870.38

Step by Step Solution

1. Step 1: Write the proportion equation
   639.75 : 870.38 = 870.38 : x
2. Step 2: Convert to fraction form
   639.75/870.38 = 870.38/x
3. Step 3: Cross multiply
   639.75x = 870.38 × 870.38
4. Step 4: Simplify right side
   639.75x = 757,552.64
5. Step 5: Solve for x
   x = 757,552.64/639.75
6. Step 6: Final answer
   x = 1,184.14