זמן, כסף והשתמש במחשבון

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.

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).

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.

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.

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.

Crossing midnight and time zones

• A flight leaves Friday 21:50 and lasts 5 h 25 min: add 5 h to reach Saturday 02:50, then 25 min to reach 03:15.
• If the destination is 3 h behind, subtract 3 h: local arrival is Saturday 00:15. A bus at 01:10 leaves 55 min later. Always state the day as well as the clock time.

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