What is the tangent ratio for ∠a?

For this case we have to define trigonometric relations of rectangular triangles, that the tangent of an angle is given by the leg opposite the angle on the leg adjacent to the angle. Then, according to the figure we have:

Books is the 3 in the ratio, so for every 3 books sold there are 4 magazines and 9 cards.

Divide the number of books by 3:

45/3 = 15

Multiply the ratio of magazines and cards by 15:

Magazines = 15 x 4 = 60

Cards = 15 x 9 = 135