Finding the Fourth Proportional to 128.72, 177.59 and 78.91

Step by Step Solution

1. Step 1: Write the proportion equation
   128.72 : 177.59 = 78.91 : x
2. Step 2: Convert to fraction form
   128.72/177.59 = 78.91/x
3. Step 3: Cross multiply
   128.72x = 177.59 × 78.91
4. Step 4: Simplify right side
   128.72x = 14,013.38
5. Step 5: Solve for x
   x = 14,013.38/128.72
6. Step 6: Final answer
   x = 108.87