Finding the Fourth Proportional to 100.78, 138.98 and 78.91

Step by Step Solution

1. Step 1: Write the proportion equation
   100.78 : 138.98 = 78.91 : x
2. Step 2: Convert to fraction form
   100.78/138.98 = 78.91/x
3. Step 3: Cross multiply
   100.78x = 138.98 × 78.91
4. Step 4: Simplify right side
   100.78x = 10,966.76
5. Step 5: Solve for x
   x = 10,966.76/100.78
6. Step 6: Final answer
   x = 108.82