Thankfully, my maths brain does this type of question on sight, but many people have lost the ability to "visualise" the problem/numbers without mechanical aids, possibly through the lack of teaching the basic 'rithmatic at an early age, (not that I'm denigrating the teaching profession - however I feel that in the current target-driven curiculum market, the foundation blocks of numeracy/literacy have been downgraded as "inappropriate to current teaching methods").
But then again I may be getting old and set in my ways, ie 1+1=2 (or used to!), but may be now 1+1 = x (loads of child support & lack of motivation to further child development).
Apologies, if the above post offends the PC, but it's my view, and I stand by it.