Mrs Tham bought \( z \) bottles of oil at \( \$ 7 \) each. She gave the cashier \( \$ 50 \). (a) Find the change Mrs Tham received in terms of \( z \). (b) If \( z=3 \), how much change did Mrs Tham receive?

To solve the problem, we will first calculate the total cost of the bottles of oil and then determine the change Mrs. Tham received. (a) The total cost of \( z \) bottles of oil, each costing \( \$ 7 \), is given by: \[ \text{Total cost} = 7z \] Mrs. Tham gave the cashier \( \$ 50 \). Therefore, the change she received can be calculated as: \[ \text{Change} = \text{Amount given} - \text{Total cost} = 50 - 7z \] Thus, the change Mrs. Tham received in terms of \( z \) is: \[ \text{Change} = 50 - 7z \] (b) Now, if \( z = 3 \), we can substitute \( z \) into the change formula: \[ \text{Change} = 50 - 7(3) \] Calculating the total cost for 3 bottles: \[ \text{Total cost} = 7 \times 3 = 21 \] Now, substituting this back into the change formula: \[ \text{Change} = 50 - 21 = 29 \] Therefore, if \( z = 3 \), Mrs. Tham received \( \$ 29 \) in change.