Finding the Fourth Proportional to 1,139.55, 1,572.58 and 642.20

Step by Step Solution

1. Step 1: Write the proportion equation
   1,139.55 : 1,572.58 = 642.20 : x
2. Step 2: Convert to fraction form
   1,139.55/1,572.58 = 642.20/x
3. Step 3: Cross multiply
   1,139.55x = 1,572.58 × 642.20
4. Step 4: Simplify right side
   1,139.55x = 1,009,908.55
5. Step 5: Solve for x
   x = 1,009,908.55/1,139.55
6. Step 6: Final answer
   x = 886.23