Finding the Fourth Proportional to 1,391.89, 1,920.78 and 770.89

Step by Step Solution

1. Step 1: Write the proportion equation
   1,391.89 : 1,920.78 = 770.89 : x
2. Step 2: Convert to fraction form
   1,391.89/1,920.78 = 770.89/x
3. Step 3: Cross multiply
   1,391.89x = 1,920.78 × 770.89
4. Step 4: Simplify right side
   1,391.89x = 1,480,710.09
5. Step 5: Solve for x
   x = 1,480,710.09/1,391.89
6. Step 6: Final answer
   x = 1,063.81