Algebra 2 Honors

I have a question on my math homework.
The problem is: Suppose r varies jointly as s and t. If s = 20 when r = 140 and t = -5, find s when r = 7 and t = 2.5.

I used r = kst. I plug in and get 140 = k(20)(5). I then multiply and get 140 = k(-100). I then divide each side by -100 and get k = -7/5. I replug and get an equation of 7 = -7/5(2.5)s. I multiple this and get 7 = -3.5s. I divide each side by -3.5 and get s = -2.

My final answer is s = -2, I just want to know if this is correct.

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  1. you can get that without worrying about k.

    since r = kst, r/st = k and is constant. So, you need s in

    140/(-5*20) = 7/(2.5s)
    s = -2
    as you calculated

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