Two numbers have difference of 0.7 and a sum of 1. what are the numbers?

We can solve this by setting up 2 equations. We can use any letters like. For now, I'll use x and y.
So now we know that    x - y = 0.7   and    x + y = 1.
Now we can eliminate one of the letters by adding or subtracting one equation from the other. I am going to eliminate y by adding the two together (the y and the -y cancel).
This gives us   2x = 1.7,    and so x = 1.7 ÷ 2 = 0.85
Finally, we can substitute x for 0.85 back into one of the original equations to figure out what y equals. I'm going to use   x + y = 1.
So now we have    0.85 + y = 1,    so y = 1 - 0.85 = 0.15

The numbers, therefore, are 0.85 and 0.15 (you can check by using the other equation).

Hope this helps!