Finding the Fourth Proportional to 20,431.72, 30,656.94 and 12,128.20

Step by Step Solution

1. Step 1: Write the proportion equation
   20,431.72 : 30,656.94 = 12,128.20 : x
2. Step 2: Convert to fraction form
   20,431.72/30,656.94 = 12,128.20/x
3. Step 3: Cross multiply
   20,431.72x = 30,656.94 × 12,128.20
4. Step 4: Simplify right side
   20,431.72x = 371,813,499.71
5. Step 5: Solve for x
   x = 371,813,499.71/20,431.72
6. Step 6: Final answer
   x = 18,197.86