Finding the Fourth Proportional to 428.52, 642.69 and 139.89

Step by Step Solution

1. Step 1: Write the proportion equation
   428.52 : 642.69 = 139.89 : x
2. Step 2: Convert to fraction form
   428.52/642.69 = 139.89/x
3. Step 3: Cross multiply
   428.52x = 642.69 × 139.89
4. Step 4: Simplify right side
   428.52x = 89,905.90
5. Step 5: Solve for x
   x = 89,905.90/428.52
6. Step 6: Final answer
   x = 209.81