Finding the Fourth Proportional to 128.56, 177.40 and 78.81

Step by Step Solution

1. Step 1: Write the proportion equation
   128.56 : 177.40 = 78.81 : x
2. Step 2: Convert to fraction form
   128.56/177.40 = 78.81/x
3. Step 3: Cross multiply
   128.56x = 177.40 × 78.81
4. Step 4: Simplify right side
   128.56x = 13,980.89
5. Step 5: Solve for x
   x = 13,980.89/128.56
6. Step 6: Final answer
   x = 108.75