How many two–digit numbers are divisible by 3?

Explanation

2 digit no divisible by 3 are `12,15,18,21.....,99`

let them be `a_1, a_2 , a_3...... `and so on

`a_2-a_1= 15-12=3`

`a_3-a_2=18-15=3`

so,`a=12 & d=3`

and we also know last term of series `=99`

now,`a_n= a+(n-1)d`

`99=12+(n-1)3`

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