Finding the Fourth Proportional to 85.91, 128.72 and 71.39

Step by Step Solution

1. Step 1: Write the proportion equation
   85.91 : 128.72 = 71.39 : x
2. Step 2: Convert to fraction form
   85.91/128.72 = 71.39/x
3. Step 3: Cross multiply
   85.91x = 128.72 × 71.39
4. Step 4: Simplify right side
   85.91x = 9,189.32
5. Step 5: Solve for x
   x = 9,189.32/85.91
6. Step 6: Final answer
   x = 106.96