Maths Paper 1, November 2024, question 12.3

A company makes 4-character codes in the form letter | digit | letter | digit. The first digit is chosen from 1 to 9 and the last digit must be an even digit (0, 2, 4, 6 or 8). Letters may be repeated. At present only 20 of the 26 letters of the alphabet may be used, which gives 18 000 different codes. The company now allows 5 of the 6 letters that were left out to be used as well, so 25 letters are available. Calculate the percentage increase in the number of different codes.