Question

Reena climbs up 5 stairs every second and then climbs down 2 stair over the next second. how many seconds will she take to climb 60 stairs​

Answer 1

Time taken by Reena to climb 60 stairs = 39 seconds

Step-by-step explanation:

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

⇒ Reena climbs (5 - 2) stairs in total after (1 + 1) seconds

⇒ Reena climbs 3 stairs in total after 2 seconds

⇒ Rate at which Reena climbs = 1 stair per 2/3 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.

⇒ Time taken by Reena to climb 60 stairs = 40 - 1 = 39 seconds

Answer 2

$\huge \mathfrak \red{answer}$

39th second

$\sf{every \: second \: reena \: climbs \: up \: 5stairs}$

$\sf{ = + 5}$

$\sf \red{she \: climb \: down \: 2stairs \: in \: the \: next}$

$\sf{ = - 2}$

$\sf \blue{in \: two \: second \: she \: climb \: 5 - 2 = 3}$

$\sf \red{in \: 38 \: second \: she \: will \: climb \: down}$

$\sf \red{ \frac{38 \times 3}{2} = 57stair}$

39 seconds she will climb down 5 more stairs

$\sf \red{57 + 5 = 62}$

$\tt{in \: 39 \: second \: she \: will \: climb \: 60 \: seocnds}$