Finding the Fourth Proportional to 428.25, 642.42 and 139.69

Step by Step Solution

1. Step 1: Write the proportion equation
   428.25 : 642.42 = 139.69 : x
2. Step 2: Convert to fraction form
   428.25/642.42 = 139.69/x
3. Step 3: Cross multiply
   428.25x = 642.42 × 139.69
4. Step 4: Simplify right side
   428.25x = 89,738.04
5. Step 5: Solve for x
   x = 89,738.04/428.25
6. Step 6: Final answer
   x = 209.55