A shipment of 205 Otter Box covers for i-phones has just arrived. The total invoice (excluding tax, shipping, etc.) is $ 4,829.75

The Otter Box "Defender" cover is priced at $39.95

The Otter Box "Case" cover is priced at $18.95

How many Otter Box "Defender" series covers are in the shipment?

Price times count of each item equals the $4829.75.

So, 39.95 x Defender + 18.95 times Case = the total invoice.

We know,

Defenders + Case = 205 (total) ... i. e.,

$39.95 + $18.95 = $ 4,829.75

that means,

no. of Defenders = Total - Case

Making it into two equations,

160 x 18.95 + 45 x 39.95 = 4829.75