Finding the Fourth Proportional to 139.30, 209.15 and 78.55

Step by Step Solution

1. Step 1: Write the proportion equation
   139.30 : 209.15 = 78.55 : x
2. Step 2: Convert to fraction form
   139.30/209.15 = 78.55/x
3. Step 3: Cross multiply
   139.30x = 209.15 × 78.55
4. Step 4: Simplify right side
   139.30x = 16,428.73
5. Step 5: Solve for x
   x = 16,428.73/139.30
6. Step 6: Final answer
   x = 117.94