Mix Mountain — Lesson 66: Carrying over several rods: 99 + 1
Carry practice →Lesson 66: Abacus 99 + 1: carrying over several rods
Level 5, Mix Mountain. When a rod is full, its ten goes to the next rod. And if that rod is full too?
What your child does, card by card
Carrying over several rods. 99 + 1 = 100. PlayType opens the lesson: “Watch the ten roll along the rods, like dominoes.”
99 + 1 = 100: ones full. Tens full. On to the hundreds! Yellow marks the ones rod, the tens rod and the hundreds rod while the kids talk about them. The abacus makes 99, then clears. Here the hands lead: they show 99, and a moment later the beads copy it. Each label lights in turn: “Ones full: 9 off”, “Tens full: 9 tens off” and “1 hundred on”. After the show, the child can touch: “The ten rolls along, like dominoes.”
95 + 5 = 100: the twins make a ten. The tens rod is full. The beads show 95 and go back to zero. “Top bead off”, “9 tens off” and “1 hundred on” glow on the card in step with the beads. The card ends on a touch: “So the ten rolls on to the hundreds.”
100 − 1 = 99: no ones. No tens. Borrow the hundred. A yellow line goes round the hundreds rod, the tens rod and the ones rod as the kids name them. 100 appears on the beads, then the rod clears again. Above, “1 hundred off”, “9 tens on” and “9 ones on” take turns to glow. Its words: “It comes back as 9 tens and 9 ones.”
Try it puts a green ring on each next bead; your child works out 99 + 1, 95 + 5, 100 - 1 and 100 - 5.
A tiny practice with no ring: work out 98 + 2, 100 - 2, 199 + 1, 97 + 3 and 101 - 2, each round starting from its own number.
In your head: the abacus is covered and your child works out 99 + 1, 100 - 1 and 98 + 2, picking each answer from 99, 100 and 101.
You're ready! A ten can roll over many rods.
What this lesson trains
12 answers, roughly five minutes: 43 moves of the beads, 18 of them with the top bead and 25 with bottom beads. Up to 200, read and set from the left. Then 3 quick looks in your head. 9 of the 9 sums need a big friend (“+1 = −9 +10”, “+5 = −5 +10”, “−1 = −10 +9”, “−5 = −10 +5”, “+2 = −8 +10”, “−2 = −10 +8” and “+3 = −7 +10”); the others use free beads.
It starts with doing: work out 99 + 1, 95 + 5, 100 - 1 and 100 - 5.
Fingers and hands
Below the abacus the drawn hands show zero and 90, 95, 97, 98 and 99 here, finger for bead: the left hand counts the tens, the right hand the ones.
+1 on the fingers: the right hand lets 9 go and the left hand opens one more finger for the ten (−9 +10). +5 on the fingers: the right hand lets 5 go and the left hand opens one more finger for the ten (−5 +10). −1 on the fingers: the left hand closes one finger for the ten and the right hand shows 9 more (−10 +9). −5 on the fingers: the left hand closes one finger for the ten and the right hand shows 5 more (−10 +5). +2 on the fingers: the right hand lets 8 go and the left hand opens one more finger for the ten (−8 +10). −2 on the fingers: the left hand closes one finger for the ten and the right hand shows 8 more (−10 +8). +3 on the fingers: the right hand lets 7 go and the left hand opens one more finger for the ten (−7 +10).
Two hands hold up to 99, so past 99 they fold and fade with a note (“Our hands stop at 99. The beads go on.”) while the beads carry on.
On “99 + 1 = 100” 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.
What PlayBuddy and PlayType say
PlayType: “Watch the ten roll along the rods, like dominoes.”
PlayBuddy: “Ninety-nine, and one more. No room again! So one bead goes up on the next rod.”
PlayBuddy: “Our hands stop at ninety-nine. The beads go on.”
PlayType: “One hundred off. Nine tens on. Nine ones on. Ninety-nine!”
Stars
12 answers: 1 slip still gives three stars; up to 3, two stars.