First off: When you are adding 10% to the £30 you are getting 99% of the price, because £30 is only 90% of the full price, and 10% of 90% = 9%. 90% + 9% = 99%. That is why you are slightly off target!

(1) You need to first say that £30 is 90%, as it is. When you receive your £30, you are only receiving 90% of the money, because the Tourist Information took 10%.

(2) So you need to find and add an extra 10%, which you would do by diving the 90% by 9 as 90/9 = 10%. So £30 divided by 9 is 30/9, which is simplified to 10/3, which is 3.333333333....recurring.

(3) Simply add this to your £30 and get £33.33. Now when you remove 10% you will remove 3.3333... and have your 90% again which is £30.