Column Tower — Lesson 74: Column sums with two-digit numbers

What your child does, card by card

Column sums with two-digit numbers. Tens first on every line. PlayType opens the lesson: “First the tens, and then the ones.”

First a reminder (“34 + 25 − 18”): the beads build 34, 84, 54, 59, 9, 49 and 41, ring by ring, while your child watches. + 25: add 2 tens, then 5 ones. − 18: take 1 ten, then 8 ones.

Try it: your child works out the columns 34 + 25 - 18 and 56 + 28 + 14, with a green ring on the next bead to move.

Then a short practice without rings: work out the columns 41 + 36 - 25 and 67 - 38 + 29, each round starting from its own number.

In your head: the abacus is covered and your child works out 20 + 30 + 10, 50 - 20 + 10 and 30 + 40 - 20, picking each answer from 40, 50 and 60.

You're ready! Two-digit columns: tens first, every line.

What this lesson trains

11 answers in about five minutes. The tasks take 28 bead moves in all: 11 with a top bead and 17 with bottom beads. The largest is 98: tens and ones. Then 3 quick looks in your head. 7 of the 8 sums need a friend (“+2 = +5 −3”, “−1 = −5 +4”, “+8 = +3 −5 +10”, “+4 = +5 −1”, “+3 = +5 −2”, “−3 = −5 +2”, “−8 = −10 +2” and “+9 = −1 +10”); the others use free beads.

It starts with doing: work out the columns 34 + 25 - 18 and 56 + 28 + 14.

The sum sheet

The lower window shows the column as a child would write it on paper: one number under another, the current line in yellow, ticks on the lines done. The abacus carries the running total from line to line.

What PlayBuddy and PlayType say

PlayBuddy: “One line at a time, top to bottom.”

Stars

11 answers: 1 slip still gives three stars; up to 2, two stars.