Finding the Third Proportional to 391.78 and 619.78

Step by Step Solution

1. Step 1: Write the proportion equation
   391.78 : 619.78 = 619.78 : x
2. Step 2: Convert to fraction form
   391.78/619.78 = 619.78/x
3. Step 3: Cross multiply
   391.78x = 619.78 × 619.78
4. Step 4: Simplify right side
   391.78x = 384,128.80
5. Step 5: Solve for x
   x = 384,128.80/391.78
6. Step 6: Final answer
   x = 980.47