Finding the Fourth Proportional to 139.89, 209.51 and 85.67

Step by Step Solution

1. Step 1: Write the proportion equation
   139.89 : 209.51 = 85.67 : x
2. Step 2: Convert to fraction form
   139.89/209.51 = 85.67/x
3. Step 3: Cross multiply
   139.89x = 209.51 × 85.67
4. Step 4: Simplify right side
   139.89x = 17,948.72
5. Step 5: Solve for x
   x = 17,948.72/139.89
6. Step 6: Final answer
   x = 128.31