Finding the Fourth Proportional to 1,223.56, 1,687.80 and 780.76

Step by Step Solution

1. Step 1: Write the proportion equation
   1,223.56 : 1,687.80 = 780.76 : x
2. Step 2: Convert to fraction form
   1,223.56/1,687.80 = 780.76/x
3. Step 3: Cross multiply
   1,223.56x = 1,687.80 × 780.76
4. Step 4: Simplify right side
   1,223.56x = 1,317,766.73
5. Step 5: Solve for x
   x = 1,317,766.73/1,223.56
6. Step 6: Final answer
   x = 1,076.99