Finding the Fourth Proportional to 21,387.61, 31,855.88 and 20,428.15

Step by Step Solution

1. Step 1: Write the proportion equation
   21,387.61 : 31,855.88 = 20,428.15 : x
2. Step 2: Convert to fraction form
   21,387.61/31,855.88 = 20,428.15/x
3. Step 3: Cross multiply
   21,387.61x = 31,855.88 × 20,428.15
4. Step 4: Simplify right side
   21,387.61x = 650,756,695.02
5. Step 5: Solve for x
   x = 650,756,695.02/21,387.61
6. Step 6: Final answer
   x = 30,426.81