Lesson 19a: Adding and subtracting tens on an abacus
Level 2, Easy Street. The tens rod adds like the ones rod.
What your child does, card by card
Adding and taking away tens. 20 + 10, 40 − 10. PlayBuddy opens the lesson: “Big numbers have tens and ones. Let's add both.”
20 + 10 = 30: 2 + 1 = 3. So 2 tens + 1 ten = 3 tens. A yellow line goes round the ones rod and the tens rod as the kids name them. 2 and 20 appear on the beads, then the rod clears again. On the card, “2 + 1 = 3” and “20 + 10 = 30” light up one by one. Then the card invites a touch: “Same move, one rod to the left.”
20 + 50 = 70: 2 + 5 = 7. So 2 tens + 5 tens = 7 tens. Yellow marks the ones rod and the tens rod while the kids talk about them. The abacus makes 2 and 20, then clears. Here the hands lead: they show 7 and 70, and a moment later the beads copy each one. Each label lights in turn: “2 + 5 = 7” and “20 + 50 = 70”. After the show, the child can touch: “The top bead on the tens rod is 50.”
40 − 10 = 30: 4 − 1 = 3. So 4 tens − 1 ten = 3 tens. The ones rod and the tens rod are outlined in yellow while the kids explain. The beads show 4 and 40 and go back to zero. Here the hands lead: they show 3 and 30, and a moment later the beads copy each one. “4 − 1 = 3” and “40 − 10 = 30” glow on the card in step with the beads. The card ends on a touch: “Same move, one rod to the left.”
Try it: your child works out 20 + 10, 20 + 50, 40 - 10, 10 + 30 and 70 - 50, with a green ring on the next bead to move.
Then a short practice without rings: work out 20 + 20, 30 + 60, 40 - 30, 90 - 60 and 50 + 40, each round starting from its own number.
Copy me: the hand in the lower window shows 30, 70 and 90, one at a time, and your child makes the same number on the beads.
Then the beads hide and the sums 20 + 10, 40 - 10 and 20 + 50 are done in the head, the answer shown on the fingers.
You're ready! Tens add and take away just like ones.
What this lesson trains
16 answers, roughly five minutes: 17 moves of the beads, 6 of them with the top bead and 11 with bottom beads. The largest is 90: tens and ones. Then 3 quick looks in your head.
It starts with doing: work out 20 + 10, 20 + 50, 40 - 10, 10 + 30 and 70 - 50.
Fingers and hands
Below the abacus the drawn hands show zero and 2, 3, 4, 7, 10, 20, 30, 40, 50, 70 and 90 here, finger for bead: the left hand counts the tens, the right hand the ones.
On “20 + 50 = 70” 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.
The fingers lead in Copy me (30, 70 and 90); the beads answer.
What PlayBuddy and PlayType say
PlayBuddy: “Big numbers have tens and ones. Let's add both.”
PlayBuddy: “To add, push beads to the bar.”
PlayType: “Two tens and one more ten make three tens. That is thirty!”
PlayType: “To take away, move beads off the bar.”
Stars
16 answers: 1 slip still gives three stars; up to 4, two stars.