Please answer! what prescription for 20/40 visual acuity?

Depends: I am guessing that your vision uncorrected is 20/40 (not too bad). The prescription to correct this depends upon how it got there. You could be myopic, hyperopic or have some degree of astigmatism. Each of these would be a different prescription.. See your ophthalmologist to get the proper one.

Depends: As a rule of thumb, the strength of the prescription needed to correct 20/40 vision is about one diopter. It may be one diopter of correction for nearsightedness, farsightedness, astigmatism, or a mix of astigmatism with nearsightedness or farsightedness. Your eye doctor will perform a refraction to determine the exact prescription and an exam to rule out a medical cause for blurred vision.