Finding the Fourth Proportional to 86.42, 128.71 and 80.80

Step by Step Solution

1. Step 1: Write the proportion equation
   86.42 : 128.71 = 80.80 : x
2. Step 2: Convert to fraction form
   86.42/128.71 = 80.80/x
3. Step 3: Cross multiply
   86.42x = 128.71 × 80.80
4. Step 4: Simplify right side
   86.42x = 10,399.77
5. Step 5: Solve for x
   x = 10,399.77/86.42
6. Step 6: Final answer
   x = 120.34