Chemistry unit 1 AS.

I came across this questions and I really cannot understand how they got to the answer they did.
In my opinion, and following the rule, you should put the mass of the desired product over the total mass of reactants. So I did and got 58%. However, the mark scheme implies differently.
An ultrasound imaging agent has the formula C4F10
It can be made by the reaction of butane and fluorine as shown in the following
equation.
C4H10 + 10F2→C4F10 + 10HF
Calculate the percentage atom economy for the formation of C4F10 in this reaction.
Give your answer to three significant figures.
...............................................................................
the answer is
238 x 100
438

= 54.3%