Question 62750
Let the number of standard delivery packages be = x

let the number of express delivery package = y

Then x + y = 120 

==> y = 120 - x ---------(1) [adding -x to either side]

Cost of x standard delivery package = $3.50x

Cost of y express delivery package = $7.50y

==> 3.5x +  7.5y = 596  ---------(2)
  
Plugging in the value of y in the above we get..

3.5x + (7.5)(120 - x) = 596

==> 3.5x + 900 - 7.5x = 596

==> - 4x + 900 = 596
==> -4x + 900 - 900 = 596 - 900 [adding - 900 to both the sides]

==> - 4x = - 304

==> - 4x/-4 = - 304/-4

==> x = 76

So y = 120 - x
     = 120 - 76

    = 44

Number of standard delivery packages = 76

Number of express delivery packages = 44


Good Luck!!!