We could work out the Z-score for each Xi giving us Zi.
Z1=(X1-16)/4, Z2=X2-7, Z3=(X3-9)/2, Z4=(X4-13)/3. The total time T to complete the errands, including walking, = X1+X2+X3+X4, Xi=μi±σiZi.
We need a critical value for Z such that the probability of exceeding this value is 1%. When Z=2.3263 approx, p=99%.
We are considering Xi>μi+σiZ.
Therefore X1>25.31, X2>9.33, X3>13.65, X4>19.98, so T=68.26 min, that is, 1hr 8.26min. Therefore t=11:08.26. There is a 1% probability that she will be back later than 11:08.26 am.