Reena climbs up five stairs every second and than climb down two stairs over the next second .how many seconds will she take to climb 60 stairs?

Given: Reena climbs up $5$ stairs every second and then climbs down $2$ stairs over the next second.

To do: To find the time taken to climb in $60$ seconds.

$\Rightarrow$ Reena climbs $( 5-2)$ stairs in total after $( 1+1)$ seconds

$\Rightarrow$ Reena climbs $3$ stairs in total after $2$ seconds

$\Rightarrow$ Time to climb $3$ stairs$=\frac{2}{3}$ seconds

$\Rightarrow$ Time taken to climb $60$ stairs$=\frac{2}{3}\times60=40$ seconds

But, when she climbs the last 5 stairs then she will reach the $60$ stairs already so she will not have to climb down the next second so, as a result saving one second of climbing.

$\Rightarrow$ Time taken by Reena to climb 60 stairs$=40-1=39$ seconds