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