16) -9+-4=-13 because in addition of 2 negative numbers always comes out negative
17) -7+10=3 because as a negative number when adding a positive number you are simply reducing the dept
18)17+-11= 6 because negative and positive mixed together is negative so that will be simply 17-11 which is 6
19)-4+-8= -12 using what I said in number 18 that will be -4-8 this will be =-12 because for example you are are owing a bank -4 then you stared owing -8 dollars now in sum gives you-12