Final answer:
To find the change Isaac should get after buying 17 anti-pollution masks, multiply the cost of each mask by the number of masks and subtract the total cost from the amount paid. Isaac should get £10.75 in change.
Step-by-step explanation:
To calculate the amount of change Isaac should get, we need to first find the total cost of the 17 anti-pollution masks. Since each mask costs £5.25, the total cost of the masks is £5.25 * 17 = £<<5.25*17=89.25>>89.25
Next, we need to find the total amount paid by Isaac. He pays with five £20 notes, so the total amount paid is 5 * £20 = £<<5*20=100>>100.
To find the change Isaac should get, we subtract the total cost of the masks from the total amount paid: £100 - £89.25 = £<<100-89.25=10.75>>10.75
Therefore, Isaac should get £10.75 in change.