How to rectify "time lost " by pendulum clock by alteration in its length? question:
If a clock loses $5$ seconds per day ,what is the alteration required in the length 
of pendulum in order that the clock keeps correct time 
$(a)\dfrac{4}{86400} $times its original length be shortened
$(b)\dfrac{1}{86400}$ times its original length be shortened
$(c)\dfrac{1}{8640}$ times its original length be shortened
$(d)\dfrac{4}{8640} $times its original length be shortened
my attempt:
there are $86400$ seconds in a day 
but clock is slow so it only counts $86395$ seconds 
so, the factor by which clock is slow is $\dfrac{86395}{86400}=0.99994212$
so, new pendulum's length should be $\left(\dfrac{86395}{86400}\right)^2\times$(original length)
but none of the options are matching my answer 
please tell me right approach to solve this problem 
thank you!

$$1-\left(\dfrac{86395}{86400}\right)^2 = 1-\left(1-\dfrac{5}{86400}\right)^2 = 1-\left(1-\dfrac{10}{86400}+\dfrac{5^2}{86400^2}\right) = \dfrac{1}{8640}-\dfrac{1}{298598400}$$ which suggests to me that you might be expected to give answer $(b)$
