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Statistics hypothesis please so lost on this one

US citizens spend an average of 2.5 hours per day talking and/or texting on cell phones with a standard deviation of 0.7 hours. A sample of 35 teens aged 15-18 years showed an average of 2.9 hours of cell phone use per day. Can we conclude that teens in this age group spend more time on cell phone then adults at a =.05 level of significance
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H0: µ=2.5
Ha: µ>2.5
We know the population standard deviation σ, so use the z-test:
z = (2.9-2.5)/(0.7/√35) = 3.381
P = 3.617*10-4
Since P<0.05, the result is significant at the 0.05-level, so reject H0.