Finding the Fourth Proportional to 139.61, 209.42 and 81.89

Step by Step Solution

1. Step 1: Write the proportion equation
   139.61 : 209.42 = 81.89 : x
2. Step 2: Convert to fraction form
   139.61/209.42 = 81.89/x
3. Step 3: Cross multiply
   139.61x = 209.42 × 81.89
4. Step 4: Simplify right side
   139.61x = 17,149.40
5. Step 5: Solve for x
   x = 17,149.40/139.61
6. Step 6: Final answer
   x = 122.84