I don't understand how to get the answer for B

Question

A programmer plans to develop a new software system. In planning for the operating system that he will​ use, he needs to estimate the percentage of computers that use a new operating system. How many computers must be surveyed in order to be 95​% confident that his estimate is in error by no more than two percentage points?

A. Assume that nothing is known about the percentage of computers with new operating systems.

Answer: n= 2401

B. Assume that a recent survey suggests that about 85​% of computers use a new operating system.

n=

Answer 1

n = z²_α/2p(1-p)/ME² For α = .05, z_α/2 = 1.96, The greatest n is needed when p = .5. Then, n=

z²_α/2*1/4/ME²

For ME = 2%, z²_α/2*1/4/ME² = 1.96² * 1/4/(.02²) = 2401

If p = .85, we have z²_α/2p(1-p)/ME² = 1.96² * .85*(1-.85)/(.02²) = 1224.51, which we round up to 1225