Finding the Fourth Proportional to 1,211.98, 1,818.12 and 390.31

Step by Step Solution

1. Step 1: Write the proportion equation
   1,211.98 : 1,818.12 = 390.31 : x
2. Step 2: Convert to fraction form
   1,211.98/1,818.12 = 390.31/x
3. Step 3: Cross multiply
   1,211.98x = 1,818.12 × 390.31
4. Step 4: Simplify right side
   1,211.98x = 709,630.42
5. Step 5: Solve for x
   x = 709,630.42/1,211.98
6. Step 6: Final answer
   x = 585.51