Can someone check my pre-calc question?

Question

smileyxl3 · Answer

A certain calling card gives the following rate to call Malaysia. There is a connection fee of $0.39 which allows you to connect for up to (but not including) one minute. For each additional minute or part of a minute, there is a $0.24 cent charge. (For example, 1 minute up to but not including 2 minutes costs $0.63.) This fare is only valid for up to (but not including) 40 minutes.

Express this information as a function, where f(x) represents the amount of money charged and x represents the number of minutes which have elapsed.

I wrote f(x) =0.39+0.24([[x]])

anonymous · Answer

I agree :)

john_es · Answer

Agree. Also, include the condition, 
$$0\leq x<40$$
I put the equal, because the company will make you pay even if you don't talk any minute (so you pay for the conecction only).

smileyxl3 · Answer

@JuanitaM Thanks! And please give a medal to @John_ES :)

john_es · Answer

No need. @JuanitaM just revised the problem before I do it. ;), but thanks anyway.