Sheet 6 of 10 · Maths
Speed, distance, time and measures
The speed triangle, metric units, the 24-hour clock and money. 8 rules, each with a worked example.
Speed, distance and time
The speed triangle
S = D ÷ T · D = S × T · T = D ÷ S
- Cover the one you want: D is on top, S and T are underneath
Example120 km in 2 hours: S = 120 ÷ 2 = 60 km/hMake the units match
Speed in km/h? Time must be in hours: minutes ÷ 60
- 15 min = 0.25 h · 30 min = 0.5 h · 45 min = 0.75 h · 20 min = ⅓ h
- 60 km/h = 1 km every minute · 30 km/h = 500 m every minute
Example40 km/h for 30 minutes: D = 40 × 0.5 = 20 kmHours as a decimal
Minutes equals decimal part of an hour × 60
Example90 km at 60 km/h: T = 90 ÷ 60 = 1.5 h = 1 hour 30 minutesTip: 1.5 hours is 1 hour 30 minutes, NOT 1 hour 50 minutes.
Metric conversions
Length
1 km = 1000 m · 1 m = 100 cm · 1 cm = 10 mm
Example2.45 m = 245 cm = 2,450 mmMass and capacity
1 kg = 1000 g · 1 litre = 1000 ml
- Big unit → small unit: multiply · Small unit → big unit: divide
Example3.2 kg = 3,200 g · 750 ml = 0.75 litres
Time and money
24-hour clock
From 1 pm onwards, add 12 to the hour
- Always 4 digits: 9:05 am = 09:05 · midnight = 00:00 · noon = 12:00
Example4:45 pm = 16:45 · 21:30 = 9:30 pmTime differences
Count on: to the next whole hour, then whole hours, then the minutes
- 60 seconds = 1 minute · 60 minutes = 1 hour · 24 hours = 1 day
Example08:45 to 11:20: 15 min + 2 h + 20 min = 2 h 35 minTip: Don't subtract times like ordinary numbers: 1120 − 845 = 275 is wrong.
Money
£1 equals 100p · write pounds with 2 decimal places
Example£5 − £2.37 = £2.63 · 3 × 45p = 135p = £1.35Tip: Working out change? Count up from the price to the amount paid.
Want the full explanation?
The study guide goes through each rule step by step, with more examples.