Question

Maths specialist ans. this​

Answer 1

a=15km

d=-1km

Sn=54km

n=?

Sn=n/2(2a+(n-1)d)

54=n/2(2*15+1-n)

108=n(31-n)

108=31n-n²

n²-31n+108=0

n²-27n-4n+108=0

n(n-27)-4(n-27)=0

(n-4)(n-27)=0

n-4=0===>n=4

n-27=0===>n=27

since n=27 will be the addition of (-)ve values

n=4hrs