If Katya has a total of 40 pairs of socks, how many pairs of white socks does she have?

(you gave the full question in the comments, hopefully this answers your question)

First, let's answer the "What percentage of Katya’s total sock collection is made up of white socks?" question:

       Our ratio of black to white is 3:7. If we add them together (3 + 7 = 10) then for every 10 socks Katya has 3 black and 7 white. We can write this as:

White = 7/10

Black = 3/10

       The question is asking for white socks, so we can drop the black and just work with the white:

White = 7/10

       Now a percentage is out of 100, and we can multiply the fraction by 10 to get a denomintaor of 100:

70 / 100 = 70%

               So,

What percentage of Katya’s total sock collection is made up of white socks?

70%

       Next, let's answer the "If Katya has a total of 40 pairs of socks, how many pairs of white socks does she have?" question:

If Katya has 40 pairs of socks then 70% of those socks will be white. We can do the following:

40 x .7 = 28

       You know how up above I said "for every 10 socks Katya has 3 black and 7 white"

Well, we have 4 tens. 7 x 4 = 28, so this also checks out! :D

If Katya has a total of 40 pairs of socks, how many pairs of white socks does she have?

28 pairs

Sorry that this is long, hopefully it helps!