Which equation represents this situation?

ivy bought some paper. she used 12 sheets. she has 11 sheets left.

how many sheets of paper did ivy buy?

a.
□ + 11 = 12

b.
□ – 12 = 11

c.
12 – □ = 11

d.
12 – 11 = □

The answer would be B. Because the square represents the unknown value of how much paper she bought. So you subtract 12 because she used that much paper. Then 11 goes on the other side because that's how much paper she has left. 

And she bought 23 sheets of paper because 23-12=11. Or 12+11= 23.