This tough brain teaser is designed to put your cognitive skills and focus to the test – and only those with a high IQ are said to be able to spot the odd letter amongst the Xs and Ks in just 13 secondsThis tough brain teaser is designed to put your cognitive skills and focus to the test – and only those with a high IQ are said to be able to spot the odd letter amongst the Xs and Ks in just 13 seconds