The SUTEC ST-991ES PLUS is a very widely used scientific calculator, and it's not hard to see why. For a very low price it delivers exceptional functionality. In fact, in this price range, you would either have to be unaware of its capabilities, or exceedingly stupid, to use anything else. In most cases it's almost certainly the former, because most students who do own this calculator have no idea of most of the features that it offers them: features that might help them get a better exam grade.
10+2 Digits Scientific Calculator,
Display Expression Same as Textbook
Extra Large Display
Two Way Power (Solar and battery)
High quality suitable for students and engineering computing