We want 85% of the calls to be answered within the new time specification, so we must first find the the z-score z0 such that P(z≤z0)=0.85. From a table or using a calculator you find z0=1.036. Now use
z = (x-µ)/σ
to find the corresponding wait time:
1.036 = (x-210)/40,
which gives x=251.45 s.
Therefore, 85% of calls should be answered within a maximum wait time of 251.45 s.