Serena is paid a base salary of $620 per month plus a commission of 9% for each of cellphones that she sells. A cellphone costs $250. If her total income for last month was $2,577.50, how many cellphones did she sell?

cellphones

Commission of a cellphone: $250 × 9% = $22.50

Total of commission: $2,577.50 – $620 = $1,957.50

Sales of cellphones: $1,957.50 ÷ $22.50 = 87 cellphones