Answer:
1 balloon= $1
1 banner=$2
if u go by ths logic: the $5 you spent and the $7 ur friend spent makes sense. if they bought 1 banner so, u remove $2 from the $7 spent, then that would leave $5 for the 5 balloon that was bought. and for the one balloon u bought, so remove $1 from the 5 you spent, that would leave you with $4 divide that by 2 and u get $2 per banner