Finding the Fourth Proportional to 139.71, 209.34 and 128.31

Step by Step Solution

1. Step 1: Write the proportion equation
   139.71 : 209.34 = 128.31 : x
2. Step 2: Convert to fraction form
   139.71/209.34 = 128.31/x
3. Step 3: Cross multiply
   139.71x = 209.34 × 128.31
4. Step 4: Simplify right side
   139.71x = 26,860.42
5. Step 5: Solve for x
   x = 26,860.42/139.71
6. Step 6: Final answer
   x = 192.26