aptitude

question; you have two flask one having 3 litre capacity  and the other having 4 litre capacity and you need to show a way to get 2 litre water out of these two

such a questions can be solved within 20 second by modeling the situation mathematically:
 let us consider we have two input 3 and 4 and with arithmatic you need to get 2 (in arithmatic we  are allwed to perform only additon and substraction because it is easy to visualise  these in real life...)

now 4-3=1 ,now we have have generated one but we cant do 1+1=2 because we have generated only one 1 . so....3-1=2 is valid so we got the required answer 2.
                                                                                              convert it into physical situation, subtraction means you are pouring one fill into other unfill and addition means you are pouring filled into other filled. so 4-3 suggest either you pour 4 litre into three or three litre into 4 litre ,since 4 litre cant be poured into 3 so we have to pour 3 into 4 litre flask. then 3-1. when you poured  3 into 4 litre flask 4 litre flask became 1 litre flask but here you cant pour 1 litre into three litre (common sense as it contains already 3 litre and has capacity of 4 litre) so you need to pour 3 litre into 1 litre(that is 4 litre flask) so the water left in the flask is 3-1=2 litre your required answer