Two cups of lemonade amount to $2 x 2 = $<<2*2=4>>4.
Two sandwiches amount to $2.50 x 2 = $<<2.5*2=5>>5.
So, the total amount that Roxanne had to pay was $4 + $5 = $<<4+5=9>>9.
Thus, her change must be $20 - $9 = $<<20-9=11>>11.
#### 11