Good solutions sesamemochi
, but I'd like to modify it slightly to make it faster to solve. From my experience, I would say that we must always try to form an equation in the variable that we need to calculate. This will save us one step at the end. For example, I would solve this problem as below -
Assume the current price to be x. So, as per the given information, we can form the below equation -
120/x = 120/(x+1) + 10
Solving this equation for x, we get 3 and -4
Choose 3 as price cannot be negative.
Secondly, for your alternate 'value plugging' approach, time can be saved if only the options with consecutive prices are considered. In this case 1, 2, 3, 4. But since 12 is not in the sequence, there was no need to even consider option E.
Lastly, in most cases, I find plugging of values a risky affair. Even plugging requires thinking, for example in this case even after plugging the values, only the results of consecutive price pairs matter, such as 1,2 ; 2,3 and 3,4. And imagine if option D was 5 instead of 4, then the plugging would have been useless. Moreover if someone does all the plugging and calculations and later realizes that there are no consecutive pairs to consider, it will be such a waste of time.
My 2 cents - All in all, I suggest that we use value plugging as the last resort. It is more important to know the concepts. Value plugging needs thinking too, so better apply thoughts to applying the concepts.