Finding the Fourth Proportional to 1,388.10, 1,915.74 and 1,315.20

Step by Step Solution

1. Step 1: Write the proportion equation
   1,388.10 : 1,915.74 = 1,315.20 : x
2. Step 2: Convert to fraction form
   1,388.10/1,915.74 = 1,315.20/x
3. Step 3: Cross multiply
   1,388.10x = 1,915.74 × 1,315.20
4. Step 4: Simplify right side
   1,388.10x = 2,519,581.25
5. Step 5: Solve for x
   x = 2,519,581.25/1,388.10
6. Step 6: Final answer
   x = 1,815.13