Finding the Fourth Proportional to 2,660.12, 3,990.78 and 811.40

Step by Step Solution

1. Step 1: Write the proportion equation
   2,660.12 : 3,990.78 = 811.40 : x
2. Step 2: Convert to fraction form
   2,660.12/3,990.78 = 811.40/x
3. Step 3: Cross multiply
   2,660.12x = 3,990.78 × 811.40
4. Step 4: Simplify right side
   2,660.12x = 3,238,118.89
5. Step 5: Solve for x
   x = 3,238,118.89/2,660.12
6. Step 6: Final answer
   x = 1,217.28