A survey was conducted by a website on which online music channels subscribers use on a regular basis. The following
information summarizes the answers.
10 listened to rap, heavy metal, and alternative rock.
13 listened to rap and heavy metal.
16 listened to heavy metal and alternative rock.
26 listened to rap and alternative rock.
37 listened to rap.
30 listened to heavy metal.
49 listened to alternative rock.
18 listened to none of these three channels.

a. How many people were surveyed?
b. How many people listened to either rap or alternative rock?
c. How many listened to heavy metal only?