Finding the Fourth Proportional to 1,139.76, 1,572.96 and 642.40

Step by Step Solution

1. Step 1: Write the proportion equation
   1,139.76 : 1,572.96 = 642.40 : x
2. Step 2: Convert to fraction form
   1,139.76/1,572.96 = 642.40/x
3. Step 3: Cross multiply
   1,139.76x = 1,572.96 × 642.40
4. Step 4: Simplify right side
   1,139.76x = 1,010,469.50
5. Step 5: Solve for x
   x = 1,010,469.50/1,139.76
6. Step 6: Final answer
   x = 886.56