Philip is downloading applications (apps) and songs to his tablet. He downloads $7$ apps and $6$ songs. Each song takes an average of $0.8$ minutes longer to download than each app. If it takes $21.7$ minutes for his downloads to finish, which of the following systems could be used to approximate $a$, the average number of minutes it takes to download one app, and $s$, the average number of minutes it takes to download one song? [[☃ radio 1]]