Time, money and using a calculator
| English | Chinese | Pinyin |
|---|---|---|
| 24-hour clock | 24小时制 | 24 xiǎo shí zhì |
| exchange rate | 汇率 | huì lǜ |
The train that left at 14 47
- Your train departs at 14 47 and the journey takes 2 hours 35 minutes. What time do you arrive?
- $14\!:\!47 + 2\!:\!35$: minutes first ($47 + 35 = 82 = 1\text{ h } 22\text{ min}$), then hours ($14 + 2 + 1 = 17$). Arrival: 17 22.
- Time arithmetic is fiddly because 60 minutes ≠ 1 hour in decimal — you need to carry over.
Time, money and calculator lab
Choose the operation that matches a real measurement problem.
The 24-hour clock 24小时制
- The 24-hour clock avoids am/pm confusion: 3.15 p.m. = 15 15, midnight = 00 00, noon = 12 00.
- Add 12 to the hour for afternoon times (1 p.m. → 13, 11 p.m. → 23).
What is 3.15 p.m. on the 24-hour clock?
Add 12 hours to the afternoon time: 3.15 p.m. = 15 15.
A train leaves at 09 35 and the journey takes 2 hours 40 minutes. What is the arrival time in 24-hour format (just the hour)?
09:35 + 2:40: minutes 35+40=75=1h 15min; hours 9+2+1=12. Arrival 12:15, hour = 12.
Converting time units
- 60 seconds = 1 minute, 60 minutes = 1 hour, 24 hours = 1 day.
- For calculations, convert to decimal hours: 2 h 30 min $= 2.5$ hours (NOT $2.30$).
- $45$ min $= \dfrac{45}{60} = 0.75$ hours.
2.30 ≠ 2 hours 30 minutes. On a calculator, 2 hours 30 minutes is $2.5$, not $2.30$. $2.30$ hours = 2 hours and 18 minutes.
Entered as a decimal of an hour, what is 2 hours 30 minutes?
30 minutes is half an hour, so 2 h 30 min = 2.5 hours.
Convert 45 minutes to a decimal of an hour.
45 ÷ 60 = 0.75 hours.
On a calculator, 2.30 hours means 2 hours and 30 minutes.
2.30 hours = 2 hours and 0.30×60 = 18 minutes. 2 hours 30 minutes = 2.5 hours.
Money and currency
- Always give money to 2 decimal places: 4.8 → 4.80.
- Convert currency using the exchange rate 汇率 as a multiplier.
- Rate 1 USD = 0.92 EUR: $150$ USD $= 150 \times 0.92 = 138$ EUR.
- To convert back: divide by the rate. $138 \div 0.92 = 150$ USD.
Return trip. You buy 200 EUR at rate 0.92 (paying $200 \div 0.92 \approx 217.39$ USD). You return with 50 EUR left and convert back at rate 0.90: $50 \times 0.90 = 45$ USD. Exchange rates always cost you something.
The exchange rate is 1 USD = 0.92 EUR. How many euros is 150 USD?
150 × 0.92 = 138 EUR.
Calculator tips
- Enter time as decimal hours for speed/distance problems.
- Use brackets to control order: $(3.5 + 2.7) \times 4$, not $3.5 + 2.7 \times 4$.
- Don't round mid-calculation — keep full precision until the final answer.
The bar model works for time too: 1 hour = 60 equal parts (minutes), so 30 min = half the bar = 0.5 hours.
You've got it
- 24-hour clock: add 12 to p.m. hours (3.15 p.m. = 15 15)
- time to decimal: minutes $\div$ 60 (2 h 30 min $= 2.5$, not $2.30$)
- money to 2 d.p.; convert currency with the exchange rate as a multiplier
- never round mid-calculation — keep full precision until the end