There is a little typo on the solution. In a warmer winter you will have gas consumption reduced by 10%, that implies a reduction of 10% of sales and variable costs (the hypothesis is to consider them linearly dependant).
Talink about the Infrastructure costs, they are composed by fixed (60%) and variable(40%) costs. Applying the saving on the variable part you have:
So that the new Infrastructure costs will be 240-9.6=230.4
Does it make sense?
Feel free to contact me if you have any doubt.