Data Sufficiency

This question might seem to be having an obvious answer and indeed it has. I know almost all the concepts of Quant but fail to go from Q50 to Q51 on gmat prep and 'the' gmat. i realized that problem is not with my concepts but with my carelessness and to counter them i have maintained a log in which i list the mistakes whenever i make them but the problem still persists. For example, recently i did a DS question in which i was asked to find no. of red balls from a collection of 10 green and red balls and it was mentioned that "no of red balls" are fewer then half. using equations in option a), i got two results 3 and 7 and marked a) as INSUFFICIENT but it was SUFFICIENT.

how to deal with such errors, my gmat is in few days