Finding the Fourth Proportional to 1,139.51, 1,572.50 and 642.20

Step by Step Solution

1. Step 1: Write the proportion equation
   1,139.51 : 1,572.50 = 642.20 : x
2. Step 2: Convert to fraction form
   1,139.51/1,572.50 = 642.20/x
3. Step 3: Cross multiply
   1,139.51x = 1,572.50 × 642.20
4. Step 4: Simplify right side
   1,139.51x = 1,009,859.50
5. Step 5: Solve for x
   x = 1,009,859.50/1,139.51
6. Step 6: Final answer
   x = 886.22