Finding the Fourth Proportional to 139.89, 209.49 and 78.91

Step by Step Solution

1. Step 1: Write the proportion equation
   139.89 : 209.49 = 78.91 : x
2. Step 2: Convert to fraction form
   139.89/209.49 = 78.91/x
3. Step 3: Cross multiply
   139.89x = 209.49 × 78.91
4. Step 4: Simplify right side
   139.89x = 16,530.70
5. Step 5: Solve for x
   x = 16,530.70/139.89
6. Step 6: Final answer
   x = 118.17