Mix Mountain — Lesson 65: When the ten needs a small friend
Carrying over several rods: 99 + 1 →Lesson 65: Abacus carrying with a small friend
Level 5, Mix Mountain. The new ten goes to the tens rod. What if no bead is free there?
What your child does, card by card
When the ten needs a small friend. 47 + 8 = 55. PlayBuddy opens the lesson: “Sometimes the new ten needs a small friend too.”
7 + 8 = 15: no room for 8: 2 off, one ten on. The beads show 7 and go back to zero. Here the hands lead: they show 7, 5 and 15, and a moment later the beads copy each one. Each label lights in turn: “−2: the big friend”, “+10: one ten on” and “= 15”. After the show, the child can touch: “2 is the big friend of 8.”
40 + 10 = 50: four tens are up. No bead is free! A yellow line goes round the tens rod as the kids name it. 40 appears on the beads, then the rod clears again. Here the hands lead: they show 40, 90 and 50, and a moment later the beads copy each one. “4 tens: no bead free”, “Top bead down” and “4 tens off = 50” glow on the card in step with the beads. The card ends on a touch: “Like 4 + 1: top bead down, four beads off.”
47 + 8 = 55: 2 off the ones. Then the ten needs help. Yellow marks the tens rod while the kids talk about it. The abacus makes 47, then clears. Here the hands lead: they show 47, 45, 95 and 55, and a moment later the beads copy each one. Above, “2 off the ones”, “Top bead down” and “4 tens off = 55” take turns to glow. Its words: “Top bead down, four tens off: one more ten.”
52 − 8: the abacus makes 52, 2, 42 and 44 by itself, a green ring on each bead just before it moves, and the hands copy. One ten off: top bead up, four tens on. Then 2 on.
In Try it a green ring marks the next bead while your child works out 47 + 8, 46 + 4, 52 - 8 and 51 - 3.
The tiny practice drops the ring: work out 48 + 5, 50 - 4, 44 + 7, 23 + 9 and 53 - 5, each round starting from its own number.
Copy me: the hand in the lower window shows 55, 50 and 44, one at a time, and your child makes the same number on the beads.
Then the beads hide and the sums 47 + 8, 52 - 8 and 46 + 4 are done in the head, the answer picked from 44, 50, 55 and 65.
You're ready! When no tens bead is free: top bead down, four tens off.
What this lesson trains
15 answers, roughly five minutes: 34 moves of the beads, 16 of them with the top bead and 18 with bottom beads. Numbers reach 55, so the tens rod and the ones rod work. Then 3 quick looks in your head. The friend rules here: +8 = −2 +10, +4 = −6 +10, −8 = −10 +2, −3 = −10 +7, +5 = −5 +10, −4 = −10 +6, +7 = −3 +10, +9 = −1 +10 and −5 = −10 +5, used in 9 of 9 sums.
The first thing your child does: work out 47 + 8, 46 + 4, 52 - 8 and 51 - 3.
Fingers and hands
Below the abacus the drawn hands show zero and 5, 7, 15, 23, 32, 40, 44, 45, 46, 47, 48, 50, 51, 52, 53, 55, 90 and 95 here, finger for bead: the left hand counts the tens, the right hand the ones.
+8 on the fingers: the right hand lets 2 go and the left hand opens one more finger for the ten (−2 +10). +4 on the fingers: the right hand lets 6 go and the left hand opens one more finger for the ten (−6 +10). −8 on the fingers: the left hand closes one finger for the ten and the right hand shows 2 more (−10 +2). −3 on the fingers: the left hand closes one finger for the ten and the right hand shows 7 more (−10 +7). +5 on the fingers: the right hand lets 5 go and the left hand opens one more finger for the ten (−5 +10). −4 on the fingers: the left hand closes one finger for the ten and the right hand shows 6 more (−10 +6). +7 on the fingers: the right hand lets 3 go and the left hand opens one more finger for the ten (−3 +10). +9 on the fingers: the right hand lets 1 go and the left hand opens one more finger for the ten (−1 +10). −5 on the fingers: the left hand closes one finger for the ten and the right hand shows 5 more (−10 +5).
On “7 + 8 = 15” the hands move first and the beads follow, so your child sees that a finger and a bead move together: open a finger and a bead goes to the bar, close it and the bead goes back.
In Copy me the hand leads with 55, 50 and 44 and your child turns each into beads.
What PlayBuddy and PlayType say
PlayBuddy: “Sometimes the new ten needs a small friend too.”
PlayType: “Big friend off, one ten on.”
PlayBuddy: “Top bead down, small friend off.”
PlayBuddy: “This one is tricky. The big friend needs a small friend.”
PlayType: “Two off the ones. Now the ten: no bead is free! Top bead down, four tens off. Fifty-five!”
PlayBuddy: “One ten off, big friend on.”
Stars
With 1 or fewer slips across the 15 answers your child earns three stars; 3 or fewer, two.