Finding the Third Proportional to 405.50 and 639.50

Step by Step Solution

1. Step 1: Write the proportion equation
   405.50 : 639.50 = 639.50 : x
2. Step 2: Convert to fraction form
   405.50/639.50 = 639.50/x
3. Step 3: Cross multiply
   405.50x = 639.50 × 639.50
4. Step 4: Simplify right side
   405.50x = 408,960.25
5. Step 5: Solve for x
   x = 408,960.25/405.50
6. Step 6: Final answer
   x = 1,008.53