Finding the Fourth Proportional to 1,139.21, 1,572.15 and 641.87

Step by Step Solution

1. Step 1: Write the proportion equation
   1,139.21 : 1,572.15 = 641.87 : x
2. Step 2: Convert to fraction form
   1,139.21/1,572.15 = 641.87/x
3. Step 3: Cross multiply
   1,139.21x = 1,572.15 × 641.87
4. Step 4: Simplify right side
   1,139.21x = 1,009,108.06
5. Step 5: Solve for x
   x = 1,009,108.06/1,139.21
6. Step 6: Final answer
   x = 885.80