I don't know about hard
est, but I thought the Merchant Taylors specimen papers (
http://www.mtsn.org.uk/admissions/specimen-entrance-papers/) looked pretty hard. Consider this from their 11+ Official Practice Paper (
http://www.mtsn.org.uk/fileadmin/content/MTSN/_Admissions/2008_11plus_EntranceExamExamplePaper.pdf):
19. Given that a = 2, b = -3 and c = 4, find the value of the following expressions:
(a) 5a + 2c
(b) ½a² - b
(c)
3ac___2b³
Each is just worth just 1 mark. In (b) you're
subtracting a negative number (hence adding a positive number) - how many 10-year olds have been taught that? In (c) you're
cubing a negative number (so it's sign flips to positive as you square it and then back to negative as you cube it). For a 10-year old? Too much.
(I hope the above had rendered correctly - I had to be a little creative to get the denominator on (c) to indent correctly.)