How effective is such strategy given the time constraint? I mean you are not able or have no time for exact solution but you must be quite sure that is, for example, c or d, not a or b. Then simply approximate that must be b given that is near but not exact solution. May such tactics be a reserve strategy for about 25 % of total Qs? Experiences of those who survived? I'm quite sure that it happens when you started with wrong kind of compounding (aka using discrete instead of continuously because you didn't read question well and no time to revert back etc).