Number of hairs in day 1 , a = 5.
Every day number of hairs increases by , d = 4.
So, it is an Arithmetic Progression with first term a = 5 and common difference d = 4.
So, number of hair in nth days is given by :
[tex]a_n=a+(n-1)d\\\\a_n=5+(n-1)4[/tex]
Let, after x days number of hairs are 73.
[tex]73=5+(x-1)4\\\\x-1=\dfrac{73-5}{4}\\\\x=18\ days[/tex]
Therefore, Zider is 18 days old.
Hence, this is the required solution.
