[Math] Probability that product of digits of a number is divisible by 3

probability

A four digit number is randomly picked from all the 4-digit numbers, then the probability that the product of its digits is divisible by 3 is ____

I am not sure if my approach is correct. Please suggest the correct method if the following approach is wrong.

My attempt: Exhaustive number of such 4 digit numbers (1000 to 9999) is 9000. If the product of its digits has to be divisible by 3, then atleast one of the 4-digits has to be one among the following set {3,6,9}.

So the probability that atleast one of the four digits has to be a multiple of 3 is $$ = (1 – None \space of \space the \space digits \space is \space a \space multiple \space of \space 3)$$ $$=\left( 1 – \frac{6}{9}.(\frac{7}{10})^3 \right)$$ $$=0.77133$$

Best Answer

0 is divisible by 3, so you must include that as well. This gets a little tricky for the thousands place though.

It should be $$1-\left(\frac{6}{9}\cdot\left(\frac{6}{10}\right)^3\right)=1-\frac{18}{125}=\frac{107}{125}=0.856$$