Finding the Third Proportional to 400.31 and 640.69

Step by Step Solution

1. Step 1: Write the proportion equation
   400.31 : 640.69 = 640.69 : x
2. Step 2: Convert to fraction form
   400.31/640.69 = 640.69/x
3. Step 3: Cross multiply
   400.31x = 640.69 × 640.69
4. Step 4: Simplify right side
   400.31x = 410,480.47
5. Step 5: Solve for x
   x = 410,480.47/400.31
6. Step 6: Final answer
   x = 1,025.40