Finding the Fourth Proportional to 78.72, 117.21 and 77.91

Step by Step Solution

1. Step 1: Write the proportion equation
   78.72 : 117.21 = 77.91 : x
2. Step 2: Convert to fraction form
   78.72/117.21 = 77.91/x
3. Step 3: Cross multiply
   78.72x = 117.21 × 77.91
4. Step 4: Simplify right side
   78.72x = 9,131.83
5. Step 5: Solve for x
   x = 9,131.83/78.72
6. Step 6: Final answer
   x = 116.00