Finding the Fourth Proportional to 1,388.72, 1,916.78 and 891.51

Step by Step Solution

1. Step 1: Write the proportion equation
   1,388.72 : 1,916.78 = 891.51 : x
2. Step 2: Convert to fraction form
   1,388.72/1,916.78 = 891.51/x
3. Step 3: Cross multiply
   1,388.72x = 1,916.78 × 891.51
4. Step 4: Simplify right side
   1,388.72x = 1,708,828.54
5. Step 5: Solve for x
   x = 1,708,828.54/1,388.72
6. Step 6: Final answer
   x = 1,230.51