June rings the same church bell for weekday weddings. The time that she spends, in minutes, ringing the bell may be modelled by the distribution \(\mathrm { N } \left( \mu , 2.4 ^ { 2 } \right)\).
Given that 80 per cent of the times that she spends ringing the bell are less than 15 minutes, find the value of \(\mu\).
[0pt]
[4 marks]
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