Finding the Fourth Proportional to 100.55, 138.78 and 56.91

Step by Step Solution

1. Step 1: Write the proportion equation
   100.55 : 138.78 = 56.91 : x
2. Step 2: Convert to fraction form
   100.55/138.78 = 56.91/x
3. Step 3: Cross multiply
   100.55x = 138.78 × 56.91
4. Step 4: Simplify right side
   100.55x = 7,897.71
5. Step 5: Solve for x
   x = 7,897.71/100.55
6. Step 6: Final answer
   x = 78.55