Finding the Fourth Proportional to 86.28, 128.94 and 78.72

Step by Step Solution

1. Step 1: Write the proportion equation
   86.28 : 128.94 = 78.72 : x
2. Step 2: Convert to fraction form
   86.28/128.94 = 78.72/x
3. Step 3: Cross multiply
   86.28x = 128.94 × 78.72
4. Step 4: Simplify right side
   86.28x = 10,150.16
5. Step 5: Solve for x
   x = 10,150.16/86.28
6. Step 6: Final answer
   x = 117.64