Finding the Fourth Proportional to 1,210.78, 1,670.90 and 686.81

Step by Step Solution

1. Step 1: Write the proportion equation
   1,210.78 : 1,670.90 = 686.81 : x
2. Step 2: Convert to fraction form
   1,210.78/1,670.90 = 686.81/x
3. Step 3: Cross multiply
   1,210.78x = 1,670.90 × 686.81
4. Step 4: Simplify right side
   1,210.78x = 1,147,591.57
5. Step 5: Solve for x
   x = 1,147,591.57/1,210.78
6. Step 6: Final answer
   x = 947.81