Calculus is generally pretty easy to do mental arithmetic on, especially when talking about real-world situations, like estimating the acceleration of a car or something. Those could have multiple answers, but one won't apply (i.e. cars are assumed to be going forward, so negative speed/acceleration doesn't make much sense, unless braking).

Math w/ quantum states is a bit less applicable, but doing some statics in your head for determining how many samples you need for a given confidence in a quantum calculation (essentially just some stats and an integral) could fit as mental math if it's your job to estimate costs. Quantum capacity is expensive, after all...