FAST Math Grade 3 common mistakes
FAST Math Grade 3 · Understand the traps and how to avoid them.
When multiplying, the student counts the first factor as one group and skips it, giving 3 × 4 = 8 instead of 12.
The student is still building the idea of equal groups and treats counting objects as the same as counting groups; the first group gets used as the starting point rather than added.
What to do instead
Have the student draw the groups as circles and place one dot in each circle before counting, then say the multiplication sentence aloud as '3 groups of 4.' Practice with arrays so the number of rows is visibly counted as groups.
The student solves '6 × 7' by adding 6 + 7 = 13 or by adding the factors repeatedly the wrong number of times.
The student confuses the operation sign with addition and has not yet automatized multiplication facts, so the default strategy is the one practiced most recently.
What to do instead
Use a two-step routine: first circle the sign and name the operation, then choose the strategy (skip counting, array, or known fact). Build fluency with a daily 5-minute fact set covering 0–10, and connect each fact to a related fact (6 × 7 is 6 × 5 plus 6 × 2).
When solving a two-step word problem, the student performs only the first operation and writes that as the answer.
The student reads for numbers and keywords rather than for the question being asked, and stops as soon as a computation is possible.
What to do instead
Require a written 'plan' line before computing: what is asked, what is known, and how many steps. Have the student restate the question in their own words and box the final question sentence before starting.
The student ignores the remainder or reports it incorrectly, e.g., answering '7 remainder 2' when the question asks how many full boxes are needed for 30 items packed 4 per box.
The student treats the remainder as a decoration rather than as information that changes the answer, and has not learned that the context decides whether to round up or drop it.
What to do instead
Teach three remainder question types explicitly: how many are left, how many full groups, and how many groups are needed. For each, ask 'can we leave items out?' Have the student write a sentence answer that names the unit and justify rounding.
The student lines up digits by their left edge rather than by place value when adding or subtracting multi-digit numbers, producing errors such as 43 + 8 = 51 written as 4 + 8 in the tens column.
The student is following a visual 'start at the left' habit from reading and has not internalized that column position encodes place value.
What to do instead
Use place-value mats or grid paper so each column is a labeled column (hundreds, tens, ones). Have the student say each addend in expanded form (40 + 3, 8) before combining, and check with an estimate.
When regrouping to subtract, the student subtracts the smaller digit from the larger in each column regardless of order, e.g., computing 52 − 27 as 35.
The student has learned 'always subtract the smaller number' as a rule to avoid negative results and does not understand borrowing as exchanging one ten for ten ones.
What to do instead
Demonstrate with base-ten blocks: trade one ten for ten ones physically, then record the change above the number. Pair every subtraction with an addition check (35 + 27 should equal 52) so the error is caught immediately.
The student reads a fraction such as 3/4 as 'three out of four parts of a whole' but cannot place it on a number line or compare it with 1/2 without drawing a pie.
Instruction focused on area models only, so the student's fraction concept is tied to shading shapes rather than to a number with a position and size.
What to do instead
Practice partitioning number lines into equal parts and labeling fractions, then compare fractions using the same whole and benchmark 1/2. Ask 'is 3/4 closer to 0, 1/2, or 1?' to build magnitude sense.
When finding the area of a rectangle, the student counts only the squares along the top and bottom edges or adds length + width + length + width.
The student confuses area with perimeter because both are introduced with the same rectangle drawing and the formulas are memorized without meaning.
What to do instead
Have the student tile the rectangle with unit squares and count all of them, then connect the count to rows × columns. Give mixed tasks that ask for area and perimeter side by side, requiring the unit to be written as square units for area.
The student reads a scaled bar graph as if each picture or bar segment counted as one, so a bar reaching 3 intervals on a scale of 5 is read as 3.
The student attends to the number of marks rather than to the key or axis labels, a habit formed from unscaled pictographs in earlier grades.
What to do instead
Before answering, have the student state the scale aloud ('each line is 5') and write the value of each gridline on the axis. Practice with two graphs using different scales and ask comparison questions that require the scale.
The student answers a multi-step problem with a number only, omitting units and the equation, and loses credit on constructed-response items even when the computation is right.
The student treats the task as arithmetic practice rather than communication and has not seen the scoring expectations for written responses.
What to do instead
Use a fixed response frame: equation with a symbol for the unknown, work shown, then an answer sentence with units. Practice scoring sample responses against a simple rubric so the student can identify what a complete answer contains.