Finding the Fourth Proportional to 2,100.78, 3,151.65 and 639.96

Step by Step Solution

1. Step 1: Write the proportion equation
   2,100.78 : 3,151.65 = 639.96 : x
2. Step 2: Convert to fraction form
   2,100.78/3,151.65 = 639.96/x
3. Step 3: Cross multiply
   2,100.78x = 3,151.65 × 639.96
4. Step 4: Simplify right side
   2,100.78x = 2,016,929.93
5. Step 5: Solve for x
   x = 2,016,929.93/2,100.78
6. Step 6: Final answer
   x = 960.09