Atumor in a rat responds to a new radiation treatment by shrinking 5% with each treatment. how many treatments before the tumor has shrunk to a quarter of its original size?

A quarter of its original size would be 25% of 100%, right? Therefore, the tumor would have to shrink 75%. If each treatment, the tumor shrinks 5% that would mean it takes 15 treatments to shrink 75 percent. 

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There is no graph but what i can say is that 1 is the slope and that -2 is the y- intercept