Question

Starting from 200 ft​ away, your friend skates toward you and then passes you. She skates at a constant speed of 20 ​ft/s. Her distance d from you in feet after t seconds is given by d=l200-20tl. At what times is she 140 ft from​ you?

Answer 1

Differentiate d

`\\ \sf\longmapsto (d)/(dx)200-20t`

`\\ \sf\longmapsto 20t`(Velocity)

Now

20t-140=0[/tex]

`\\ \sf\longmapsto 20t=140`

`\\ \sf\longmapsto t=(140)/(20)`

`\\ \sf\longmapsto t=7s`