Lindsay sells copy machines which come in standard and enhanced models. the standard model sells for $210 and the enhanced model sells for $265. if lindsay sold a total of 19 units and took in $4,485, how many of the standard model did she sell?

You can write two equations in the two unknowns. Let s and e represent the numbers of Standard and Enhanced units sold, respectively. Then the problem statement tells you
.. s + e = 19 . . . . . . . . . . . . . . . Lindsay sold a total of 19 units
.. 210s +265e = 4485 . . . . . . Lindsay took in $4,485

Since you want to know s, you can use the first equation to write an expression for e in terms of s.
.. e = 19 -s
Substitute this into the second equation.
.. 210s +265(19 -s) = 4485
.. -55s + 5035 = 4485 . . . . . . eliminate parentheses
.. -55s = -550 . . . . . . . . . . . . . subtract 5035
.. s = 10 . . . . . . . . . . . . . . . . . . . divide by the coefficient of s

Lindsay sold 10 Standard models.