SME
Grade 9 Math · Problem type
Solve a formula for a letter
Use this for “Solve P = 2l + 2w for w.”
Diagram
This is an example. Type your own values to start a new problem.
P (cm)
=
2
×
l (cm)
+
2
×
w (cm)
Formulas
P
=
2
×
l
+
2
×
w
30 = 2 × 9 + 2 × 6
Assumptions
•
Solve for the letter first, then put the numbers in last.
•
Undo the operations on w in reverse order: take 2l away, then divide by 2.
•
All three lengths use the same unit.
Step-by-step
WE KNOW
l = 9 cm, P = 30 cm
FIND
width (w)
Step 1 · Find width (w)
Use P = 2 × l + 2 × w
Take
2
l
from
both
sides,
then
divide
both
sides
by
2:
w
=
P
− 2
l
2
.
w
=
P
− 2 ×
l
2
w
=
30 − 2 × 9
2
w
=
30 − 18
2
w
=
12
2
w
=
6
cm
Check
Put the numbers back in. Both sides should match.
30 = 2 × 9 + 2 × 6 ✓
Topic check
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