I had a similar problem before, but I solved it. Here is how I solved it:
Look over all your mistakes in the test, then figure out what was wrong.
Correct EVERY mistake. Then tell your big brother/dad/mum to make you another math test.
Then keep practicing on your mistakes until you finally understand the principle.
If there is something you don't understand, then I prefer you to go to your dad/mum/big brother or even your teacher to get that solved. Or you can ask Google if none is available

:ps: This strategy made me go from 3 to a 5. I am freakin' happy. (1 = fail ---- 6 = best grade )