Answer

Sheena bought:

a sandwich: 6.95$ and 2 pretzels at 2.75$ each.

So the total amount that sheena had to pay:

6.95 + (2 x 2.75) = 6.95 + 5.5 = 12.45$

Sheena paid with 20$ bills, so the change will be:

20 - 12.45 = 7.55$