Question 1175386
z=(x-mean)/sigma/sqrt(n)
< (15-28)/9.002/sqrt(70)
<-13*sqrt(70)/9.002
<-12.078
probability is essentially 0 meaning 0 calls will be responded to in < 15 minutes.
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The wording isn't clear after that.  How many will be responded to in 50% of what?  If it is 50% of the calls are responded to, using the word approximately, 14 of the 28. 
100%, mathematically, infinite time. Realistically, greater than 3 std errors, a SE being sd/sqrt(n) or 9.002/sqrt(70)=1.076, so 3 SE  are 3.228 min or waiting time of 31.32 minutes.