What you need to know

Things to remember:

  • Substitution just means replacing a letter with a number.
  • We don’t write \times when there is a number before a letter, so you need to remember that it is hiding there!
    • xy=x\times y
  • A fraction is another way of writing a division question.

We have looked at substituting for a single variable before, so let’s have a quick look back at how we did that first.

 

Use substitution to find the value of 5x when x = 13

5x=5\times x=5\times13=65

Divisions questions could either appear as a division:

Use substitution to find the value of x\div3 when x = 9

 

x\div3=9\div3=3

 

Or as a fraction that we have to change:

Use substitution to find the value of \frac{20}{x} when x = 5

\frac{20}{x}=20\div x=20\div5=4

When we are substituting for two variables we do it in the exact same way, we just have to substitute for two variables!

Use substitution to find the value of x+y when x = 13 and y = 11

 

x+y=13+11=24

 

Use substitution to find the value of x-y when x = 4 and y = 9

 

x-y=4-9=-5

 

Use substitution to find the value of xy when x = 13 and y = 12

 

xy=x\times y=13\times 12= 156

 

Use substitution to find the value of x\div y when x = 72 and y = 9

 

x\div y= 72\div9=8

 

Use substitution to find the value of \frac{x}{y} when x =64 and y = 8

 

Hint: Remember, a fraction is just another way of writing a division.

 

\frac{x}{y}=x\div y= 64\div8=8

Example Questions

xy=x\times y=7\times 11= 77

\frac{y}{x}=y\div x= 35\div7=5

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