Finding the Fourth Proportional to 85.98, 128.80 and 78.98

Step by Step Solution

1. Step 1: Write the proportion equation
   85.98 : 128.80 = 78.98 : x
2. Step 2: Convert to fraction form
   85.98/128.80 = 78.98/x
3. Step 3: Cross multiply
   85.98x = 128.80 × 78.98
4. Step 4: Simplify right side
   85.98x = 10,172.62
5. Step 5: Solve for x
   x = 10,172.62/85.98
6. Step 6: Final answer
   x = 118.31