What Causes Concentration And Memory Problems With Juxtaposing Of Numbers?

I have been having some problems at school. I would do my homework or take a test and I can´t concentrate. If I have to read something it takes me a while to concentrate and get what I´m reading because I have to read it more than one time. I have also had problems with the usage of numbers. For example: on one of my tests I tried to solve a word problem but used the numbers from another problem. I will also change some numbers like 145 and put it as 541 and I would change it and put it again. This is not an everyday thing but it happens. I would want to know if there´s something wrong or if I can do something about it because it is hurting my grades.