Ashipment 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

Is that all any more to the question