I think the simplicity of the maths questions steers the brain towards a simplistic answer to the final one, "red" and "hammer" being the most obvious and basic in each category.
I think there are those among us who may dispute the "simplicity of the maths questions", Roger, and who still got the final question 'right'.
My own theory is that the hammer image comes from the scrolling down, 'hammering' each sum in quick succession (correctly or incorrectly). The red aspect may be triggered at the start, with the phrase "don't
stop to think about it", and again just before the final sum, when we are told it's "almost over", both conjuring up red traffic lights telling us to stop. "Calculations are hard work" may also sub-consciously suggest red, being a busy colour, and also the colour of 'men at work' triangular road signs.
Speaking of hard work...
