Lesson 86a: Multiplying by tens on an abacus
Level 7, Times Market. Groups of tens live on the tens rod.
What your child does, card by card
Times tens. 4 × 2 = 8. So 4 × 20 = ? PlayType opens the lesson: “Now let's try a harder one.”
4 × 2 = 8: four groups of 2. The beads work the column line by line, zero, 2, 4, 6 and 8, while the sum sheet below lights each line and ticks it; “2”, “4”, “6” and “8” light up as the total grows. Watch which beads move.
4 × 20 = 80: four groups of 20: the same beads. 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; “20”, “40”, “60” and “80” light up as the total grows. One rod to the left.
3 × 30: the abacus makes 30, 80, 60 and 90 by itself, a green ring on each bead just before it moves, and the sum sheet below ticks each line. 3 × 3 = 9. So 3 × 30 = 90.
In Try it a green ring marks the next bead while your child works out 4 × 20, 3 × 30 and 2 × 40.
3 × 40 = 120: 3 × 4 = 12. One rod to the left: 120. The beads work the column line by line, zero, 40, 80 and 120, while the sum sheet below lights each line and ticks it; “40”, “80” and “120” light up as the total grows. 12 tens are 1 hundred and 2 tens.
A tiny practice with no ring: work out 2 × 30, 4 × 20, 3 × 30, 2 × 50 and 6 × 20, each round starting from its own number.
In your head: the abacus is covered and your child works out 3 × 20, 2 × 40 and 3 × 30, picking each answer from 60, 80 and 90.
You're ready! Times tens: the small fact, one rod to the left.
What this lesson trains
About five minutes and 17 answers. Altogether the child makes 21 bead moves (6 with the top bead, 15 with bottom beads). The largest is 120: hundreds, tens and ones. Then 3 quick looks in your head. Small friends do 3 of the 11 sums, by +2 = +5 −3, +3 = +5 −2 and +4 = +5 −1.
The first thing your child does: work out 4 × 20, 3 × 30 and 2 × 40.
The sum sheet
In Column Tower the lower window is a sum sheet instead of the hands: the column written out one line under another, the line being worked in yellow, the lines done ticked, and the total once the last line is in. The beads keep the running total, so the abacus is never cleared in the middle of a column.
What PlayBuddy and PlayType say
PlayType: “Times means groups of the same number.”
PlayBuddy: “Four twos make eight. So four twenties make eighty. The same beads, one rod to the left!”
Stars
With 1 or fewer slips across the 17 answers your child earns three stars; 4 or fewer, two.