Finding the Fourth Proportional to 100.56, 138.76 and 86.78

Step by Step Solution

1. Step 1: Write the proportion equation
   100.56 : 138.76 = 86.78 : x
2. Step 2: Convert to fraction form
   100.56/138.76 = 86.78/x
3. Step 3: Cross multiply
   100.56x = 138.76 × 86.78
4. Step 4: Simplify right side
   100.56x = 12,041.59
5. Step 5: Solve for x
   x = 12,041.59/100.56
6. Step 6: Final answer
   x = 119.75