Finding the Fourth Proportional to 1,128.70, 1,691.42 and 640.72

Step by Step Solution

1. Step 1: Write the proportion equation
   1,128.70 : 1,691.42 = 640.72 : x
2. Step 2: Convert to fraction form
   1,128.70/1,691.42 = 640.72/x
3. Step 3: Cross multiply
   1,128.70x = 1,691.42 × 640.72
4. Step 4: Simplify right side
   1,128.70x = 1,083,726.62
5. Step 5: Solve for x
   x = 1,083,726.62/1,128.70
6. Step 6: Final answer
   x = 960.15