If it is 4:00 PM currently, then time in next 15:00 hrs will be

$\text{Current time}=4{:}00\ \text{PM}.$

$15\text{ hours}=12\text{ hours}+3\text{ hours}.$

$4{:}00\ \text{PM}+12\text{ hours}=4{:}00\ \text{AM (next day)}.$

$4{:}00\ \text{AM}+3\text{ hours}=7{:}00\ \text{AM}.$

$\text{Time after 15 hours}=7{:}00\ \text{AM}.$