Question

Work Backward You want to arrive at school at 7:45 A.M. It takes you half an hour to shower and get dressed, 15 minutes to eat breakfast, and 20 minutes to walk to school. What is the latest you can get up and still arrive at school on time? (p. 777).

Short answer

The latest time to get up to reach school on time at 7:45 A.M. is 6:40 A.M.

Step-by-step solution

Step-1 – Apply the concept addition

Two numbers with similar signs get added and the resulting number carries the similar sign.

Two numbers with opposite signs get subtracted and the resulting number carries the sign of the larger number.

Step-2 – Given

Time to arrive at school is 7:45a.m.

Time required to shower and get dressed is half an hour or 30 minutes $\left(T_{shower}=30\right)$

Time required to eat breakfast is 15 minutes $\left(T_{breakfast}=15\right)$

Time required to walk to school is 20 minutes $\left(T_{walk}=20\right)$

Step-3 – Explanation

Total time required for all activities are

$\begin{array}{l}{T}_{shower}+T_{breakfats}+T_{walk}\\ =30+15+20\left[\text{Substitute}\right]\\ =65\text{minutes}\left[\text{Add}\right]\end{array}$

So, 65 minutes when converted to hours and minutes it becomes 1:05

Since,

Time required to prepare before reaching school is 1:05

Therefore, subtract $1:05$from $7:45$to get the time to wake up, that subtract 1 hour from 7 hour and 5 minutes from 45 minutes $\left(45-5=40\right)$

$7:45-1:05=6:40$

Hence, the time to wake up to reach school on time is 6:40 A.M.