Hi there!
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I believe your answer is:
[tex]n = t + r[/tex]
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Here’s why:
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[tex]\boxed{\text{Solving for 'n'...}}\\\\t = n - r\\----------\\\rightarrow t + r = n -r + r\\\\\rightarrow t+r = n\\\\\rightarrow \boxed{n=t+r}[/tex]
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Hope this helps you. I apologize if it’s incorrect.