What this lesson shows
Round to friendly numbers first, then use the estimate to check the real answer.
The worked example
The lesson works this one step by step: 38 × 21. Then your kid tries one like it on paper.
The steps
- Round each number
- Multiply the round numbers
- Check the exact answer is close
Helping at home
Round each number to the nearest ten first, multiply the round numbers, then work the exact answer and check it is close.
If your kid gets stuck
Round first, then multiply.
Watch it with Lux
With Lux, this lesson is a narrated video and a printed page your kid works on. Then Lux helps with the same math on their own worksheet, one question at a time.
Standards
- Common Core 4.NBT.B.5
Multiply a whole number of up to four digits by a one-digit whole number, and multiply two two-digit numbers, using strategies based on place value and the properties of operations. Illustrate and explain the calculation by using equations, rectangular arrays, and/or area models.
Matches a standard in all 50 states and DCSee each state's code
- Alabama MA19.4.11a
- Alaska 4.NBT.5
- Arizona 4.NBT.B.5 *
- Arkansas 4.CAR.3
- California 4.NBT.B.5
- Colorado 4.NBT.B.5
- Connecticut 4.NBT.B.5
- Delaware 4.NBT.B.5
- District of Columbia 4.NBT.B.5
- Florida MA.4.NSO.2.2, MA.4.NSO.2.3, MA.4.NSO.2.5
- Georgia 4.NR.2.3
- Hawaii 4.NBT.B.5
- Idaho 4.NBT.B.5.a, 4.NBT.B.5.b
- Illinois 4.NBT.B.5
- Indiana 4.CA.1
- Iowa 4.NBT.B.5
- Kansas 4.NBT.5
- Kentucky KY.4.NBT.5
- Louisiana 4.NBT.B.5
- Maine 4.NBT.B.5
- Maryland 4.NOS.B.4, 4.NOS.B.5.a, 4.NOS.B.5.b, 4.NOS.B.5.c, 4.NOS.B.5.d, 4.NOS.B.6.a, 5.NOS.B.4.a, 5.NOS.B.5.a
- Massachusetts 4.NBT.B.5
- Michigan 4.NBT.B.5
- Minnesota 3.1.2.5, 4.1.1.3, 4.1.1.4, 5.1.1.3
- Mississippi 4.NBT.5
- Missouri 4.NBT.A.6
- Montana MT.4.NBT.5, 4.D1.21, 4.D2.4, 4.D2.9
- Nebraska 4.A.1.b, 4.A.1.d
- Nevada 4.NBT.B.5
- New Hampshire 4.NBT.B.5
- New Jersey 4.NBT.B.5
- New Mexico 4.NBT.B.5
- New York NY-4.NBT.5
- North Carolina NC.4.NBT.5
- North Dakota 4.NO.NBT.5
- Ohio 4.NBT.5
- Oklahoma 3.N.2.8, 4.N.2.2, 4.N.2.3, 5.N.2.1
- Oregon 4.NBT.B.5
- Pennsylvania CC.2.1.4.B.2
- Rhode Island 4.NBT.B.5
- South Carolina 4.NR.1.2, 5.PAFR.1.1, 4.PAFR.1.3
- South Dakota 4.NBT.B.5
- Tennessee 4.NBT.B.5
- Texas 111.5.b.4.G, 111.6.b.4.B, 111.6.b.4.C, 111.6.b.4.D, 111.6.b.4.G
- Utah 4.NBT.5
- Vermont 4.NBT.B.5
- Virginia 4.CE.2a, 4.CE.2g.i, 4.CE.2g.ii, 4.CE.2g.iii, 5.CE.1a
- Washington 4.NBT.B.5
- West Virginia M.4.10
- Wisconsin M.4.NBT.B.5, M.4.NBT.A.3
- Wyoming 4.NBT.5a, 4.NBT.5b, 4.NBT.5c, 4.NBTP.5.A, 4.NBTP.5.B, 4.NBTP.5.C