Do a counting of how many numbers are in red. There are 13 of them.

Alternatively, if you do not wish to write out every number and count the number of numbers which contain 3s, recognise the following.

Between the numbers 1 and 40, there exist numbers containing 3. We start by considering the number containing the most number of 3s.

33 has two 3s. That’s one number.

There are three other numbers apart from 33 within that range which end with 3 (03, 13, 23).

There are nine other numbers apart from 33 within that range which start with 3 (30, 31, 32, 34, 35, 36, 37, 38, 39).

In total, there are 1 + 3 + 9 = 13 numbers in the list containing the digit 3 (of which the digit 3 appears a total of 14 times since 3 appears twice in 33).

The reason why we should pick out numbers with the most 3 first is to avoid potential repetition (I mean accidental picking of the same number again). This is useful especially if the question wants you to find the total number of times the digit 3 appears (not the number of numbers containing 3) and if they give a wider range of numbers of, say, 1 to 10,000.