Question 64972
All right, so the start point is that our function is linear. 
Draw the coordinate system. 
Let the price be x and number of cups be y.
We know that when x = 1, y = 50. Plot this point. 
We also know that when x = 2, y = 25. Plot this point too. 
Draw a line between the two plotted points. 
Let's try to find the point-intercept equation of this line. Since all we know is the two points, we can make a system of two point-slope equations and find the slope. Let's do that:
point (1,50): y-50 = m(x-1); y = m(x-1) + 50
point (2,25): y-25 = m(x-2); y = m(x-2) + 25
Therefore, m = -25 
Substitute -25 for m in any of the above two equations to get:
y=-25(x-1)+50   
or
y=-25(x-2)+25 

You can now check these equations by replacing x=1 ($1/cup) and x=2 ($2/cup)...
 

Hope it helped! 
Cristiana