L1 · Multiplying natural numbers: standard algorithm
“I can multiply a 3-digit number by a 1-digit or 2-digit number using partial products and the standard algorithm - and I estimate first, so I can tell whether my product is reasonable.”
A bike costs $247. How much do 3 bikes cost? Estimate first: 247 sits just below 250 on the number line, and 250 x 3 = 750, so the product should be a little below 750. Step 1: Ones. 7 x 3 = 21. Write 1 one, regroup 2 tens onto the place-value chart. Step 2: Tens. 4 tens x 3 = 12 tens, plus the 2 regrouped tens = 14 tens. Write 4 tens, regroup 1 hundred. Step 3: Hundreds. 2 hundreds x 3 = 6 hundreds, plus the 1 regrouped hundred = 7 hundreds. So 247 x 3 = 741 - just below the estimate of 750, so it is reasonable. Model: the place-value chart showing 200 x 3 = 600, 40 x 3 = 120, 7 x 3 = 21 adding to 741, beside the estimation number line with the product landing just under 750.
In Step 2, why did we add 2 extra tens before writing the answer?
A tablet costs $348. How much do 2 tablets cost? -> 348 x 2 = 696
"156 x 4: 100 x 4 = 400, 50 x 4 = 200, 6 x 4 = 24, so 156 x 4 = 400 + 200 + ___ = ___" -> 24; 624
10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.