I see what you mean by classes of problems. Some of the problems from the above links I know how to do (although working out might take some time and a pen and

I see what you mean by classes of problems. Some of the problems from the above links I know how to do (although working out might take some time and a pen and paper) and then there are some problems where I don't know where to start. I was just never taught how to think about numbers that way even though the link says those problems are for middle school children.