Sensim Math · Depth 한국어

2-2 · Time

Compare hours first, then minutes

2.MD.C.7 · take · grade 2

Archetype: Elapsed Time and Base-Sixty Regrouping · step in a 10-type progression

▶ Practice — 8 problems

The clocks show the times that Liam, Mia, and Noah each arrived at school this morning. Which student arrived at school first?

The three clocks show these times:

12 3 6 9 Liam 12 3 6 9 Mia 12 3 6 9 Noah
Show solution

Understand

Three students arrived at school at the times shown on their clocks: Liam 8:43, Mia 9:00, Noah 8:28. Find who arrived first (the earliest time).

Givens
  • Liam's clock reads 8:43.
  • Mia's clock reads 9:00.
  • Noah's clock reads 8:28.
  • From the figure, Liam's hour hand is just past 8 with the minute hand near the 9-mark (8:43), Mia's clock shows the hour hand on 9 and minute hand on 12 (9:00), and Noah's hour hand is past 8 with the minute hand a bit past the 5-mark (8:28).
Unknowns
  • Which student arrived at school first (earliest clock time).
Constraints
  • All times are in the morning, so compare hours first, then minutes.

Plan

#1 Draw a Diagram · also uses: #2 Make a Systematic List

Read each clock face to get the time, then list the three times and compare. Compare the hour first; among the equal hours, compare the minutes to find the earliest.

Execute

#1 Draw a Diagram 2.MD.C.7
From the clock faces: Liam 8:43, Mia 9:00, Noah 8:28.
Liam 8:43, Mia 9:00, Noah 8:28\text{Liam } 8{:}43,\ \text{Mia } 9{:}00,\ \text{Noah } 8{:}28
Reading the hour and minute hands on an analog clock is a core second-grade time skill.
#2 Make a Systematic List 2.MD.C.7
Two students (Liam and Noah) arrived in the 8 o'clock hour; Mia arrived at 9 o'clock. Since 8 comes before 9, Mia is not the earliest, so the first arrival is Liam or Noah.
8<98 < 9
An earlier hour always means an earlier time, no matter the minutes.
#2 Make a Systematic List 2.MD.C.7
Liam is at 8:43 and Noah at 8:28. Since 28 minutes is earlier than 43 minutes, Noah arrived first.
28<4328 < 43
When the hour is the same, the smaller minute number is the earlier time.
Answer: Noah arrived at school first (8:28).

Review

Ordering the three times earliest to latest gives 8:28 (Noah), 8:43 (Liam), 9:00 (Mia). Noah's 8:28 is the smallest, so Noah arriving first is correct.

Convert each time to minutes after 8:00 — Noah 28, Liam 43, Mia 60 — and pick the smallest (28), which is Noah.

Standards · min grade 2

  • 2.MD.C.7 Tell and write time from analog and digital clocks to nearest five minutes — Reading the three clock faces and comparing the times to find the earliest arrival.
💡 Compare the hour first, then the minutes if hours tie: that's all Grade 2 clock-reading!