unit demand can be expressed as a function of unit cost.

y=f(x)

think of the $ and the number of units as x and y coordinates.

say that y is the price

and x is the demand

y1=$48 x1=2500

y2=$51 x2=2465

so find a linear y=f(x) equation that intercepts those two points and you will arrive at formula that will give you both answers.