Finding the Fourth Proportional to 2,100.78, 3,150.97 and 639.95

Step by Step Solution

1. Step 1: Write the proportion equation
   2,100.78 : 3,150.97 = 639.95 : x
2. Step 2: Convert to fraction form
   2,100.78/3,150.97 = 639.95/x
3. Step 3: Cross multiply
   2,100.78x = 3,150.97 × 639.95
4. Step 4: Simplify right side
   2,100.78x = 2,016,445.90
5. Step 5: Solve for x
   x = 2,016,445.90/2,100.78
6. Step 6: Final answer
   x = 959.86