Finding the Fourth Proportional to 139.42, 209.15 and 78.42

Step by Step Solution

1. Step 1: Write the proportion equation
   139.42 : 209.15 = 78.42 : x
2. Step 2: Convert to fraction form
   139.42/209.15 = 78.42/x
3. Step 3: Cross multiply
   139.42x = 209.15 × 78.42
4. Step 4: Simplify right side
   139.42x = 16,401.54
5. Step 5: Solve for x
   x = 16,401.54/139.42
6. Step 6: Final answer
   x = 117.64