The ten-frame is composed of two rows of five squares. Numbers are formed by placing dots, counters or other objects within the squares of the ten-frame, one dot per square.
As the Earth rotates through 360° in 24 hours, 15° longitude corresponds to 1 hour. Each change of 15° longitude corresponds to a natural time zone. However, time zones frequently do not align with country boundaries and not all time zones are one-hour intervals. Australia has multiple time zones. Some of them are half-hour and quarter-hour time zones.
Trading or exchanging units is used to complete a subtraction by regrouping. To subtract 15 from 42, 5 from 42 can be achieved by trading one group of 10 for 10 ones. So 42 is regrouped as 3 tens and 12 ones.