Column Tower — Lesson 76: Column sums with three-digit numbers

What your child does, card by card

Column sums with three-digit numbers. Hundreds, tens, ones on every line. PlayBuddy opens the lesson: “Big numbers work the same way. Left to right on every line.”

Watch me: 125, 425, 475, 465, 468, 463, 473, 273, 263, 253, 258 and 257 appear on the beads one move at a time, each bead ringed in green before it goes. + 348: 3 hundreds, 4 tens, 8 ones.

Try it puts a green ring on each next bead; your child works out the columns 125 + 348 - 216 and 407 + 285 + 163.

A tiny practice with no ring: work out the columns 532 - 247 + 381 and 619 + 254 - 473, each round starting from its own number.

In your head: the abacus is covered and your child works out 100 + 200 + 300, 500 - 200 + 100 and 300 + 300 - 100, picking each answer from 400, 500 and 600.

You're ready! Three-digit columns, left to right on every line.

What this lesson trains

11 answers in about five minutes. The tasks take 45 bead moves in all: 15 with a top bead and 30 with bottom beads. Up to 873, read and set from the left. Then 3 quick looks in your head. Friends do 8 of the 8 sums, by +4 = +5 −1, +8 = +3 −5 +10, −6 = −10 +5 −1, +2 = +5 −3, +5 = −5 +10, +6 = −4 +10, +3 = +5 −2, −2 = −5 +3, −4 = −10 +6, −7 = −10 +5 −2, +8 = −2 +10, +4 = −6 +10 and −4 = −5 +1.

Your child’s first moves: work out the columns 125 + 348 - 216 and 407 + 285 + 163.

The sum sheet

Below the abacus a sum sheet replaces the hands: it writes the column out, lights the line your child is on and ticks each line done; the total appears after the last line. The beads hold the running total the whole way down.

What PlayBuddy and PlayType say

PlayBuddy: “Big numbers work the same way. Left to right on every line.”

Stars

Three stars for up to 1 slip in 11 answers, two stars for up to 2.