Finding the Fourth Proportional to 2,639.69, 3,960.20 and 790.80

Step by Step Solution

1. Step 1: Write the proportion equation
   2,639.69 : 3,960.20 = 790.80 : x
2. Step 2: Convert to fraction form
   2,639.69/3,960.20 = 790.80/x
3. Step 3: Cross multiply
   2,639.69x = 3,960.20 × 790.80
4. Step 4: Simplify right side
   2,639.69x = 3,131,722.21
5. Step 5: Solve for x
   x = 3,131,722.21/2,639.69
6. Step 6: Final answer
   x = 1,186.40