If you have 5 potato chips, and I give you negative one potato chip, how many potato chips do you have? I know that it is impossible in real life, but just imagine how many potato chips you would have if I gave you negative one? That would make you have 4 potato chips.
5 potato chips plus negative 1 potato chips = 4 potato chips
5 + - 1
= 5 - 1
= 4
So...
70 + - 60
= 70 - 60
= 10