1. 13+12-10
2. 25-10
3. 15

we could write this as:

(+13)+1(+12)+1(-10)

now it is easier to understand what is going on 13 remains as it is. since positive 12 multiplies with a positive one it remains a positive 12 since positive multiplied with positive will always result in a positive answer. Next we multiply positive 10 with a negative 1. this will result in a negative 10 since a positive number multiplied with a negative number always results in a negative answer then you add and subtract them in the order given to get the final result.