Finding the Fourth Proportional to 85.76, 128.56 and 78.56

Step by Step Solution

1. Step 1: Write the proportion equation
   85.76 : 128.56 = 78.56 : x
2. Step 2: Convert to fraction form
   85.76/128.56 = 78.56/x
3. Step 3: Cross multiply
   85.76x = 128.56 × 78.56
4. Step 4: Simplify right side
   85.76x = 10,099.67
5. Step 5: Solve for x
   x = 10,099.67/85.76
6. Step 6: Final answer
   x = 117.77