Acell phone store sells the basic model of a phone for $30 and the upgraded model for $100 when a customer signs a two-year agreement. on tuesday, the store has $900 in sales. the store pays $5 for each basic model and $25 for each upgraded model. if the cost of the cell phones sold on tuesday is $200, how many upgraded models were sold? a 6 phones b 10 phones c 15 phones d 42 phones