Question

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?

Answer 1

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