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Chapter 6 · Equations · page 28 of 68

Undo in reverse order

levelGrade 7
standard7.EE.B.4
practiceTwo-step equations

To build 3x + 5, you multiply by 3 and then add 5. To take it apart, you reverse the order: subtract 5 first, then divide by 3. Like taking off shoes before socks.

How to use it

  1. Undo addition or subtraction first.
  2. Then undo multiplication or division.
  3. If the variable is inside parentheses, undo the multiplication on the parentheses first, then what is inside.

Example

3x+5=20  ⇒  3x=15  ⇒  x=53x + 5 = 20 \;\Rightarrow\; 3x = 15 \;\Rightarrow\; x = 5
2(x+3)=14  ⇒  x+3=7  ⇒  x=42(x + 3) = 14 \;\Rightarrow\; x + 3 = 7 \;\Rightarrow\; x = 4

Where it goes wrong

  • Dividing first: 3x + 5 = 20 → x + 5 = 20/3 is a mess. Subtract the 5 first.
  • Subtracting a number from only the side it is on.

Related

Two-step equationsWhat an equation isOrder of operations
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