Finding the Third Proportional to 399.56 and 517.78

Step by Step Solution

1. Step 1: Write the proportion equation
   399.56 : 517.78 = 517.78 : x
2. Step 2: Convert to fraction form
   399.56/517.78 = 517.78/x
3. Step 3: Cross multiply
   399.56x = 517.78 × 517.78
4. Step 4: Simplify right side
   399.56x = 268,097.42
5. Step 5: Solve for x
   x = 268,097.42/399.56
6. Step 6: Final answer
   x = 670.98