I am sure there must be such Apps around now. I hate that we have to do them - pilots don't need to do
quick mental maths, so the time-limited tests are a proxy for age; which they are not allowed to ask us.
Nintendo DS had games such as 'Brain Training' which have a mental maths element. It brings up a random number, then an operator and you apply that to the previous answer and scribble the new answer onto the other touch screen as fast as you can. The game recognises your writing and it speeds up as you do.
The other elements of Brain Training (and another game called Eye Training), are useful for increasing your processing speed.
And I've written programmes in my Psion to give myself pilot maths problems, e.g. how many nautical miles to climb or descend x thousand feet. Or what rate of V/S required for a given altitude change and distance.
A bit old-school, but they worked for me.
If you cannot find any Apps, you might find a Nintendo DS and Brain Training etc. at your local electronic secondhand exchange shop, e.g. CeX.
Good luck
