(((20-r τ) t-5 r τ)/(20 t+5 r t+r (t^2)))

(((20-r τ) t-5 r τ)/(20 t+5 r t+r (t^2)))/.{r->1}

Simplify[D[((t (20-τ)-5 τ)/(25 t+(t^2))),t]]

Solve[(((t^2) (-20+τ)+125 τ+10 t τ)/((t^2) (((25+t))^2)))==0,t]

Plot[((t (20-τ)-5 τ)/(25 t+(t^2)))/.τ->25,{t,0,50},PlotRange->{-1,1}]

data1=Import["/Users/troyday/Desktop/Untitled Set 1.txt","Table"]

ListPlot[{data1},PlotStyle->{Blue},Frame->True,FrameLabel->{"Travel time, T (seconds)","Surface time, t (seconds)"},LabelStyle->Directive[FontSize->14]]

Export["1.eps",%4,"EPS"]