Finding the Fourth Proportional to 139.56, 209.31 and 78.81

Step by Step Solution

1. Step 1: Write the proportion equation
   139.56 : 209.31 = 78.81 : x
2. Step 2: Convert to fraction form
   139.56/209.31 = 78.81/x
3. Step 3: Cross multiply
   139.56x = 209.31 × 78.81
4. Step 4: Simplify right side
   139.56x = 16,495.72
5. Step 5: Solve for x
   x = 16,495.72/139.56
6. Step 6: Final answer
   x = 118.20