Finding the Fourth Proportional to 1,139.78, 1,572.72 and 642.30

Step by Step Solution

1. Step 1: Write the proportion equation
   1,139.78 : 1,572.72 = 642.30 : x
2. Step 2: Convert to fraction form
   1,139.78/1,572.72 = 642.30/x
3. Step 3: Cross multiply
   1,139.78x = 1,572.72 × 642.30
4. Step 4: Simplify right side
   1,139.78x = 1,010,158.06
5. Step 5: Solve for x
   x = 1,010,158.06/1,139.78
6. Step 6: Final answer
   x = 886.27