Finding the Third Proportional to 218.81 and 385.31

Step by Step Solution

1. Step 1: Write the proportion equation
   218.81 : 385.31 = 385.31 : x
2. Step 2: Convert to fraction form
   218.81/385.31 = 385.31/x
3. Step 3: Cross multiply
   218.81x = 385.31 × 385.31
4. Step 4: Simplify right side
   218.81x = 148,465.72
5. Step 5: Solve for x
   x = 148,465.72/218.81
6. Step 6: Final answer
   x = 678.51