Finding the Fourth Proportional to 86.42, 128.87 and 81.50

Step by Step Solution

1. Step 1: Write the proportion equation
   86.42 : 128.87 = 81.50 : x
2. Step 2: Convert to fraction form
   86.42/128.87 = 81.50/x
3. Step 3: Cross multiply
   86.42x = 128.87 × 81.50
4. Step 4: Simplify right side
   86.42x = 10,501.85
5. Step 5: Solve for x
   x = 10,501.85/86.42
6. Step 6: Final answer
   x = 121.53