In order to find the constant of proportionality between t and r, as they are inversely proportional, we just need to multiply their values.
When typing 40 words per minute (that is, r = 40), you can finish in 50 minutes (that is, t = 50). So the constant is:
[tex]40\cdot50=2000[/tex]
Now, to write an equation relating t and r, we use this variables in the product above, and make their product equal the constant we found. Then, we isolate the variable t:
[tex]\begin{gathered} t\cdot r=2000 \\ t=\frac{2000}{r} \end{gathered}[/tex]
