Why the denominator stays: 4th grade math lesson

What this lesson shows

Fixes a mistake

3/8 + 4/8 is 7/8, not 7/16. Eighths plus eighths are still eighths.

The worked example

The lesson works this one step by step: 3/8 + 4/8. Then your kid tries one like it on paper.

The steps

Shade both on one bar
The parts stay the same size
Add the tops, keep the bottom

Helping at home

Say the parts out loud: 3 eighths plus 4 eighths. Draw one bar and shade both. The parts never change size, so the bottom stays the same.

If your kid gets stuck

Say the parts out loud. Do they change size?

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.

See how Lux works

Standards

Common Core 4.NF.B.3a
Understand addition and subtraction of fractions as joining and separating parts referring to the same whole.

Matches a standard in all 50 states and DCSee each state's code

Alabama MA19.4.15b
Alaska 4.NF.3.a
Arizona 4.NF.B.3.a, 4.NF.A.2.b
Arkansas 4.NPV.5, 4.CAR.5
California 4.NF.B.3
Colorado 4.NF.B.3.a
Connecticut 4.NF.B.3
Delaware 4.NF.B.3
District of Columbia 4.NF.B.3
Florida MA.4.FR.2.2
Georgia 4.NR.4.3, 4.NR.4.4, 4.NR.4.6, 5.NR.3.2
Hawaii 4.NF.B.3
Idaho 4.NF.A.2.b, 4.NF.B.3.a
Illinois 4.NF.B.3
Indiana 5.NS.1, 4.CA.6
Iowa 4.NF.A.2, 4.NF.B.3.a, 4.NF.B.3.c
Kansas 4.NF.3.a
Kentucky KY.4.NF.3.a, KY.4.NF.3.b, KY.4.NF.3.c, KY.4.NF.2
Louisiana 4.NF.B.3.a, 4.NF.B.3.b
Maine 4.NF.B.3a
Maryland 4.NOS.D.10.a, 4.NOS.E.12.a, 4.NOS.E.12.b, 4.NOS.E.12.c
Massachusetts 4.NF.B.3.a
Michigan 4.NF.B.3
Minnesota 4.1.2.2, 4.1.2.3, 5.1.2.3
Mississippi 4.NF.3.a
Missouri 4.NF.A.3, 4.NF.B.4, 5.NF.A.3
Montana MT.4.NF.3.a, MT.4.NF.3.c, 4.D1.2, 4.D2.6, 4.D4.1, 4.D5
Nebraska 4.N.2.c, 4.N.3.b, 4.N.3.c, 5.N.2.b
Nevada 4.NF.B.3
New Hampshire 4.NF.B.3
New Jersey 4.NF.B.3a
New Mexico 4.NF.B.3
New York NY-4.NF.2, NY-4.NF.3.a, NY-4.NF.3.c
North Carolina NC.4.NF.3.a
North Dakota 4.NO.NF.5
Ohio 4.NF.3.a
Oklahoma 4.N.3.2, 4.N.3.3, 4.N.3.5, 5.N.1.3
Oregon 4.NF.A.2, 4.NF.B.3
Pennsylvania CC.2.1.4.C.2
Rhode Island 4.NF.B.3.a
South Carolina 4.NR.2.4, 4.NR.2.6, 5.NR.2.1, 4.PAFR.2.1, 3.NR.2.4, 4.NR.2.5
South Dakota 4.NF.B.3.a
Tennessee 4.NF.A.2, 4.NF.B.3.a
Texas 111.6.b.3.A, 111.6.b.3.D, 111.6.b.3.E, 111.6.b.3.F, 111.6.b.3.G
Utah 4.NF.3.a, 4.NF.3.c
Vermont 4.NF.B.3
Virginia 3.NS.3b, 3.NS.3e, 4.NS.3a, 4.NS.3b, 4.NS.3c, 4.NS.3d, 4.CE.3a
Washington 4.NF.B.3
West Virginia M.4.14
Wisconsin M.4.NF.A.2, M.4.NF.B.3
Wyoming 4.NF.2a, 4.NF.2b, 4.NFP.2.A, 4.NFP.2.B, 4.NF.3a, 4.NFP.3.A

Why the denominator stays