Halfway through an undergraduate course in engineering, I'm now planning to review math fundamentals from pre-algebra, algebra, geometry to trigonometry and finally calculus because, as you may know, having a solid foundation in math is vital for any engineering course, and I've always been weak in math. I also happen to be very weak in mental arithmetic (adding, multiplying, etc. in head). Even a calculation as simple as 4+7 makes me think for many seconds, and when I can't figure the answer out I use my fingers to count! If this is for addition what can we say about subtraction? For addition involving two negative integers I always use the calculator so as not to make a mistake, even for such numbers as -7-3. Does having these problems impair one's ability to learn new math concepts? Should I improve on mental math before I start reviewing?