The extra bag cost $90.

Previously the average cost is $30. The average cost has increased to $35.

This means those bags she bought at first cost $5 more than before on average.


So out of the $90 that the extra bag cost, we subtract $35 for this extra bag first (since it's now the average price).

$90 - $35 = $55

This means the $55 of extra cost goes to the bags bought at first.

Since each bag bought at first costs $5 more than before on average,

$55 more ÷ $5 more per bag = 11

There are 11 bags bought at first.


Algebra is not required here.