Total magnification = objective magnification X ocular magnification.
The lens that a person looks into is called the ocular lens and the lens nearest the specimen (pictured) is called the objective lens.
To get the total magnification take the power of the objective (4X, 10X, 40x) and multiply by the power of the eyepiece, usually 10X.
You are given the objective magnification, take this and multiply by 10 which is the ocular/eyepice which is usually 10x
=> 4x objective view
= 4 x 10 = 40x
=> 10x objective view
= 10 x 10 = 100x
=> 40x objective view
= 40 x 10 = 400x