Finding the Fourth Proportional to 85.69, 128.88 and 78.31

Step by Step Solution

1. Step 1: Write the proportion equation
   85.69 : 128.88 = 78.31 : x
2. Step 2: Convert to fraction form
   85.69/128.88 = 78.31/x
3. Step 3: Cross multiply
   85.69x = 128.88 × 78.31
4. Step 4: Simplify right side
   85.69x = 10,092.27
5. Step 5: Solve for x
   x = 10,092.27/85.69
6. Step 6: Final answer
   x = 117.78