Jumps of the same size
Skip counting is counting in jumps of the same size, missing out the numbers in between. To count in twos, say every second number: 2, 4, 6, 8, 10. Each number is 2 more than the one before it.
Skip counting is quicker than counting in ones whenever things come in equal groups. Socks come in pairs, so 6 pairs are counted 2, 4, 6, 8, 10, 12: that is 12 socks, found in six numbers instead of twelve.
Counting in twos rings every second number: 2, 4, 6, and on up to 20.
Each step along this line is a jump of 2, so it lands only on the numbers you say when you count in twos.
Counting in fives
Counting in fives goes 5, 10, 15, 20, 25, 30. Every number in the count ends in 5 or in 0, and the two endings take turns: 5, then 10, then 15, then 20.
On a grid with 10 numbers in each row, that pattern is easy to see. The numbers you say make two straight columns, one of numbers ending in 5 and one of numbers ending in 0.
The fives make two columns: the numbers ending in 5 and the numbers ending in 0.
Each step along this line is a jump of 5.
2 jumps of 5 = 10
Count on to 50 in jumps of 5
Jumps of 5 land in the middle and at the end of each row, so the ringed numbers make two columns.
Counting in tens and hundreds
Counting in tens goes 10, 20, 30, 40, 50. Every number in the count ends in 0, so on the grid they make one column.
Bigger jumps work the same way. Counting in hundreds goes 100, 200, 300, and on to 1000, one hundred at a time.
The tens all end in 0, so they make one column.
Three jumps of 100 from 0 land on 100, 200 and 300.
Starting and stopping
A skip count does not have to start at 0. Counting in tens from 7 goes 7, 17, 27, 37, 47. Every number in that count ends in 7, because each jump adds one more ten and leaves the 7 alone.
When the last group is not full, stop skip counting and count on in ones. For 3 full bags of 5 apples and 2 loose apples, count the bags in fives, 5, 10, 15, then count on the loose apples in ones, 16, 17. There are 17 apples.
Worked example: Sheets of Five Stickers with a Last Sheet That Is Not Full
Question Siti has 7 full sheets of stickers. Each full sheet has 5 stickers. One more sheet has only 3 stickers on it. (a) How many stickers are on the 7 full sheets? (b) How many stickers does Siti have altogether?
1.Draw the 7 full sheets with 5 stickers on each. Draw the last sheet with 3 stickers.
Seven sheets are full, with 5 stickers on each. The last sheet has only 3 stickers. 2.Count the full sheets in fives: 5, 10, 15, 20, 25, 30, 35.
Count the full sheets in fives: 5, 10, 15, 20, 25, 30, 35. 3.(a) There are 35 stickers on the 7 full sheets.
(a) There are 35 stickers on the 7 full sheets. 4.(b) The last sheet is not a full five, so count on in ones: 36, 37, 38. Siti has 38 stickers altogether. Check: 35 + 3 = 38.
(b) The last sheet is not a full five. Count on in ones: 36, 37, 38. There are 38 stickers.
Answer: (a) 35 stickers; (b) 38 stickers
Common mistakes
- Counting the last sheet as another five and getting 40. The last sheet has only 3 stickers, so after 35 the count goes on in ones, not in fives.
- Counting the sheets and answering 8. The question asks for stickers, and each full sheet is 5 stickers, not 1.
Worked example: House Numbers on One Side of a Street
Question The houses on one side of a street have odd numbers. The first house is number 21. The numbers go up in twos. (a) What is the number of the fifth house? (b) Ravi lives at number 35. Which house in the row is his?
1.Draw a row of houses. Write 21 on the first house. Each next house is 2 more.
The first house is number 21. Each next house is 2 more. 2.Count on in twos to the fifth house: 21, 23, 25, 27, 29.
Count on in twos to the fifth house: 21, 23, 25, 27, 29. 3.(a) The fifth house is number 29.
(a) The fifth house is number 29. 4.Keep counting in twos until you reach 35: 31, 33, 35. These are the sixth, seventh and eighth houses.
Keep counting in twos until you reach 35: 31, 33, 35. 5.(b) Number 35 is the eighth house in the row. Every number in the row is odd, because 2 more than an odd number is odd again.
(b) Number 35 is the eighth house in the row.
Answer: (a) number 29; (b) the 8th house
Common mistakes
- Adding five twos to 21 to get 31. The first house is already number 21, so the fifth house is only 4 steps of 2 away: 21 + 8 = 29.
- Counting on in ones: 21, 22, 23. The even numbers are on the other side of the street, so this side goes up in twos.