Finding the Third Proportional to 78.50 and 128.50

Step by Step Solution

1. Step 1: Write the proportion equation
   78.50 : 128.50 = 128.50 : x
2. Step 2: Convert to fraction form
   78.50/128.50 = 128.50/x
3. Step 3: Cross multiply
   78.50x = 128.50 × 128.50
4. Step 4: Simplify right side
   78.50x = 16,512.25
5. Step 5: Solve for x
   x = 16,512.25/78.50
6. Step 6: Final answer
   x = 210.35