Two numbers have a difference of 0.7 and a sum of 1

Let one of the number be x.

They have a difference of 0.7
⇒ the other number is x - 0.7

Their sum is 1
⇒ x + (x - 0.7) = 1

Solve x:
x + (x - 0.7) = 1
x + x - 0.7 = 1
2x = 1.7
x = 0.85

x = 0.85
x - 0.7 = 0.85 - 0.7 = 0.15

The two numbers are 0.15 and 0.85