Finding the Third Proportional to 399.50 and 861.50

Step by Step Solution

1. Step 1: Write the proportion equation
   399.50 : 861.50 = 861.50 : x
2. Step 2: Convert to fraction form
   399.50/861.50 = 861.50/x
3. Step 3: Cross multiply
   399.50x = 861.50 × 861.50
4. Step 4: Simplify right side
   399.50x = 742,182.25
5. Step 5: Solve for x
   x = 742,182.25/399.50
6. Step 6: Final answer
   x = 1,857.78