Hello, Mukilab!

Your method is excellent . . . and your answer is correct!

I'll show you a method I often use.

Amy plays one game of snooker and one game of billards each Friday.

The probability that she wins every Friday is as follows:

. . Snooker: Win = 1/3

. - Billards: Win = 3/4

If she plays one game of each for a number of Fridays,

calculate how many games Amy did not win at either

if she won at both on 21 Fridays.

The probability that Amy wins both games on a particular Friday is:

. .

Since she won both games 21 times, we assume she played on 84 Fridays.

Place the information in a chart . . .

. .

We know that she wins of her snooker games.

. .

We know that she wins of her billiards games.

. .

Enter that information in our chart:

. .

Now you can supply the missiing numbers and answer the question.