Lesson 87: Two-digit times one-digit on an abacus
Level 7, Times Market. Break the big number into tens and ones. Do each part. Add them.
What your child does, card by card
Two-digit times one-digit. 23 × 4: the 20 first, then the 3. PlayType opens the lesson: “Break the big number into tens and ones.”
20 × 4 = 80: 4 twos make 8. 4 twenties make 80. The beads work the column line by line, zero, 20, 40, 60 and 80, while the sum sheet below lights each line and ticks it; “4 twos = 8” and “4 twenties = 80” light up as the total grows. The same beads, one rod to the left.
23 × 4 is 4 groups of 23: 23 + 23 + 23 + 23 = 92 The beads work the column line by line, zero, 23, 46, 69 and 92, while the sum sheet below lights each line and ticks it; “23”, “46”, “69” and “92” light up as the total grows. Right, but slow. There is a quicker way.
23 × 4 = 80 + 12: all the 20s first. Then all the 3s. The beads work the column line by line, zero, 80 and 92, while the sum sheet below lights each line and ticks it; “20 × 4 = 80”, “3 × 4 = 12” and “Together: 92” light up as the total grows. The same answer: 92.
Watch me: 90 and 93 appear on the beads one move at a time, each bead ringed in green before it goes. 30 × 3 is 90. 1 × 3 is 3. Together: 93.
Try it puts a green ring on each next bead; your child works out 23 × 4 and 32 × 3.
A tiny practice with no ring: work out 13 × 5, 24 × 3, 12 × 4 and 14 × 6, each round starting from its own number.
Then the beads hide and the sums 20 × 3, 40 × 2 and 30 × 3 are done in the head, the answer picked from 60, 80 and 90.
You're ready! Tens part, ones part, added on the beads.
What this lesson trains
About five minutes and 15 answers. Altogether the child makes 22 bead moves (8 with the top bead, 14 with bottom beads). Up to 96, read and set from the left. Then 3 quick looks in your head.
It starts with doing: work out 23 × 4 and 32 × 3.
The sum sheet
The lower window shows the column as a child would write it on paper: one number under another, the current line in yellow, ticks on the lines done. The abacus carries the running total from line to line.
What PlayBuddy and PlayType say
PlayType: “Break the big number into tens and ones.”
PlayType: “Times means groups of the same number.”
PlayBuddy: “Big part first, then the small part.”
Stars
15 answers: 1 slip still gives three stars; up to 3, two stars.