Question

How many numbers from $201$ to $250$ are divisible by $5$, but not by $3$?

Answer 1

Here again, the numbers divisible by $5$ are $205,210,215,220,225,230,235,240,245,250$. Now you compute the digital sum of these numbers: you get $7,3,8,4,9,5,10,5,11,7$. Among these, the only numbers divisible by $3$ are $3,6,9$. Thus among the $10$ numbers divisible by $5$, only three numbers are also divisible by $3$. The remaining $7$ numbers are not divisible by $3$.