Finding the Third Proportional to 399.09 and 784.69

Step by Step Solution

1. Step 1: Write the proportion equation
   399.09 : 784.69 = 784.69 : x
2. Step 2: Convert to fraction form
   399.09/784.69 = 784.69/x
3. Step 3: Cross multiply
   399.09x = 784.69 × 784.69
4. Step 4: Simplify right side
   399.09x = 615,734.47
5. Step 5: Solve for x
   x = 615,734.47/399.09
6. Step 6: Final answer
   x = 1,542.83