Question

the equation R= -0.028(T) + 20.8 can be used to predict the world record in the 200-m dash, where R is the record in seconds and T is the number of years since 1920. in what year will the record be 18.0 second?

Answer 1

$\bf R=-0.028(t)+20.8~~ \begin{cases} R=seconds\\ t=\stackrel{1920+}{years}\\ ------\\ R=18.0 \end{cases}\implies 18=-0.028t+20.8 \\\\\\ 0.028t=20.8-18\implies 0.028t=2.8\implies t=\cfrac{2.8}{0.028}\implies t=100 \\\\\\ \stackrel{\textit{since \underline{t} is years since 1920}}{1920+100\implies 2020}$