Lesson 106: Two-digit flash sums: abacus mental math
Level 9, Mind Castle. Tens first, then ones, in your head as on the beads.
What your child does, card by card
Two-digit flash sums. Two-digit numbers, one at a time. PlayType opens the lesson: “First the tens, and then the ones.”
Try it puts a green ring on each next bead; your child works out the columns 12 + 25 and 34 + 21 + 13.
Flash sums in your head: the abacus is covered and the numbers appear one at a time, 1.5 seconds each (12 + 25, 34 + 21 + 13, 45 - 12 + 20 and 23 + 31 + 15); your child adds them in the head and picks the total from 37, 53, 68 and 69.
You're ready! Two-digit flash sums, tens first.
What this lesson trains
7 answers, roughly five minutes: 8 moves of the beads, 3 of them with the top bead and 5 with bottom beads. Numbers reach 68, so the tens rod and the ones rod work. Then 4 quick looks in your head. 1 of the 3 sums need a small friend (“+2 = +5 −3” and “+1 = +5 −4”); the others use free beads.
It starts with doing: work out the columns 12 + 25 and 34 + 21 + 13.
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
PlayType: “First the tens, and then the ones.”
PlayBuddy: “Click Let's Begin, or press Enter!”
PlayType: “Now do it in your head.”
Stars
7 answers: a clean run gives three stars; up to 1, two stars.