Finding the Fourth Proportional to 85.55, 128.25 and 80.78

Step by Step Solution

1. Step 1: Write the proportion equation
   85.55 : 128.25 = 80.78 : x
2. Step 2: Convert to fraction form
   85.55/128.25 = 80.78/x
3. Step 3: Cross multiply
   85.55x = 128.25 × 80.78
4. Step 4: Simplify right side
   85.55x = 10,359.39
5. Step 5: Solve for x
   x = 10,359.39/85.55
6. Step 6: Final answer
   x = 121.10