i hv prob in the following question as i enclose the solution also plz read this and explain that how 10 is divided by 11 (10/11). thanks

Q: High/Low with stepped fixed cost

VC per unit is constant and there is a step up of 10% in the total
fixed costs when activity exceeds 5500
Units Total cost
4000 40800
6000 50000
7500 54800
What is the total cost at 5000 units?

Sol: VC = (54800 – 50000) / (7500 – 6000) = 3.20
FC (above 5500) = 54800 – (7500 @ 3.20) = 30800
FC (below 5500) = 30800 * 10/11= 28000
TC at 5000 units = 5000 @ 3.20 + 28000 = 44000