p=mq+b where p=price, q=quantity, and m and b are constants, is the demand function.
3.50=250q+b, 5.50=50q+b. Subtract one equation from the other: 2=-200q so q=-1/100=-0.01.
3.50=-2.5+b and b=6. Therefore the demand function is p=6-0.01q.
When the demand level is 115, q=115 and p=6-1.15=ghc 4.85.