so, she wants to be at school at 7.40 (5 mins early)
she needs:
15 mins
5/12 hours (which is

, so 25 mins
so far: 40 mins
then 0.7 and 0.15: we can add those, since they're in decimals: 0.85.
how many minutes is this?

so 39 mins.
together with the previous 40, it's 79 minutes, which is 1 hour and 19 minutes: so she needs to wake up 1 hour and 19 minutes before 7.40 , which is
6.21!