Once you have completed your drill, you are presented with the total number of sums completed as well as the number of right and wrong sums. How exactly are these numbers calculated?
Right sums are calculated quite easily:
If a new sum is displayed and you get it right on your first try, this sum will be calculated as "right". A different sum will now be displayed.
Wrong sums are slightly more complicated to calculate:
If a new sum is displayed and you get it wrong on the first try, this sum will be calculated as "wrong". Instead of displaying a different sum now (as with sums that are answered correctly), the same sum will be displayed and you will need to retry your answer until you eventually get it right. The aim of this is to help you try and master the sums you are struggling with (instead of just skipping over them). These "repeated" sums will not count additionally towards your total sums or wrong sums, only the initial try will count.
So, in short: if a new sum is displayed and you get it right - your "right" sums are incremented by 1. If a new sum is displayed and you get it wrong 5 times before getting the right answer - your "wrong" sums will be incremented by 1.