Given that Tickets are marked from $1$ to $20$ are mixed up. One ticket is picked at random.

to find: Probability that the ticket bears a multiple of $3$ or $7$

so, Total number of cards is $20$.

the cards marked which is multiple of $3$ or $7$ are $3,6,7,9,12,14,15$ and $18$.

Thus, the total number of cards marked multiple of $3$ or $7$ is $8$.

We know that the Probability = Number of favorable outcomes/ Total number of outcomes

Hence, the probability of drawing a card that is a multiple of $3$ or $7$ is $8/20=2/5$