. It’s because a prime factor of all even numbers is 2. Therefore the prime factors of all even squares includes two, therefore, the square must be even.
With Calculator: To work out the multiplier, take the percentage, divide by 100 and add 1 17.5% / 100 = 0.175 1 + 0.175 = 1.175 £50 x 1.175 = £58.75
Without Calculator: First work out easy percentages of 50 (if needed use previous answers (such as 10) to work out smaller percentages (such as 5%)) 100% = 50 10% = 5 5% = 2.5 2.5% = 1.25 10% + 5% + 2.5% = 17.5% 5 + 2.5 + 1.25 = £8.75 Add this on to the original price: £50 + £8.75 = £58.75