Big Friends Bridge — Lesson 54: Big friends: minus 2 and minus 1
Which way to take away? →Lesson 54: Abacus big friends: minus 2 and 1
Level 4, Big Friends Bridge. The last two big friends, taking away.
What your child does, card by card
Big friends: minus 2 and minus 1. 2 and 8 make 10. 1 and 9 make 10. PlayType opens the lesson: “Same trick, new friend.”
10 − 1: lesson 7, backwards: 9 + 1: nine beads back, one ten on. Now the other way. A yellow line goes round the tens rod and the ones rod as the kids name them. 9 appears on the beads, then the rod clears again. Here the hands lead: they show 9, 10, zero and 9, and a moment later the beads copy each one. The labels “9 + 1 = 10”, “−10: one ten” and “+9: the big friend = 9” light up as each idea comes. “Minus 1: take a ten, give 9 back.” Any bead or finger may be tapped afterwards.
11 − 2: only 1 bead on the ones rod. Yellow marks the ones rod while the kids talk about it. The abacus makes 11, then clears. On the card, “Start at 11” and “− 2? Only 1 one” light up one by one. Then the card invites a touch: “2 will not come off. So take a ten!”
Minus 2: one ten off, 8 on: 10 is 2 and 8. Taking a ten is 8 too many. On this card “−10: one ten”, “+8: the big friend” and “−2 = −10 +8” glow in turn as the beads move. Your child copies the order to work out 11 - 2 and 10 - 2.
20 − 1 = 19: the abacus makes 20, 10 and 19 by itself, a green ring on each bead just before it moves, and the hands copy. One ten off the tens rod. Nine beads on the ones rod. One ten stays.
In Try it a green ring marks the next bead while your child works out 10 - 1, 20 - 1 and 11 - 2.
The tiny practice drops the ring: work out 10 - 1, 11 - 2, 10 - 2, 20 - 1 and 30 - 1, each round starting from its own number.
Copy me: the hand in the lower window shows 9, 8 and 19, one at a time, and your child makes the same number on the beads.
Then the beads hide and the sums 10 - 1, 11 - 2 and 10 - 2 are done in the head, the answer picked from 8, 9 and 11.
You're ready! Minus 2: take a ten, give 8 back. Minus 1: take a ten, give 9 back.
What this lesson trains
16 answers in about five minutes. The tasks take 37 bead moves in all: 13 with a top bead and 24 with bottom beads. The largest is 30: tens and ones. Then 3 quick looks in your head. The friend rules here: −2 = −10 +8 and −1 = −10 +9, used in 10 of 10 sums.
It starts with doing: work out 10 - 1, 20 - 1 and 11 - 2.
Fingers and hands
Below the abacus the drawn hands show zero and 8, 9, 10, 11, 19, 20, 29 and 30 here, finger for bead: the left hand counts the tens, the right hand the ones.
−2 on the fingers: the left hand closes one finger for the ten and the right hand shows 8 more (−10 +8). −1 on the fingers: the left hand closes one finger for the ten and the right hand shows 9 more (−10 +9).
On “10 − 1: lesson 7, backwards” 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.
Copy me turns it round: 9, 8 and 19 appear on the hand first.
What PlayBuddy and PlayType say
PlayType: “Big friend off, one ten on.”
PlayBuddy: “One ten off, big friend on.”
Stars
16 answers: 1 slip still gives three stars; up to 4, two stars.