A bookstore had 45 copies of a magazine. Yesterday, it sold 2/3 of them. Today, it sold 1/3 of what remained. How many copies does the bookstore have left?

After the first day, 1/3 of the original amount remained:

... (1/3) ·45 = 15

After the second day, 2/3 of that amount remained:

... (2/3) ·15 = 10

The bookstore has 10 copies left.