[The narrator reads out the onscreen text.]

NARRATOR: Question 5. Part a. If 2x plus 5y equals 30, find x when y equals 4. Part b. If y equals 4x minus 15, find x when y equals 21.

Part a. We substitute 4 in place of y, giving 2x plus 5 times 4 equal to 30. So 2x plus 20 equals 30. Subtracting 20 from both sides gives 2x equals 10. And dividing both sides of the equation by 2 gives x equals 5. So therefore, if 2x plus 5y equals 30, x is equal to 5 when y equals 4.

[Part b - If y = 4x - 15, find x when y = 21.]

NARRATOR: Part b. Again, substitute 21 in place of y, which gives 21 equal to 4x minus 15. Adding 15 to both sides gives 36 equals 4x. And dividing both sides of the equation by 4 gives 9 equal to x, which we'll rewrite as x equals 9. So if y is equal to 4x minus 15, then x is equal to 9 when y equals 21.