Finding the Fourth Proportional to 100.82, 138.78 and 86.91

Step by Step Solution

1. Step 1: Write the proportion equation
   100.82 : 138.78 = 86.91 : x
2. Step 2: Convert to fraction form
   100.82/138.78 = 86.91/x
3. Step 3: Cross multiply
   100.82x = 138.78 × 86.91
4. Step 4: Simplify right side
   100.82x = 12,061.37
5. Step 5: Solve for x
   x = 12,061.37/100.82
6. Step 6: Final answer
   x = 119.64