It's quite simple if you can understand how an equation works.
First, you get to interpret the given data.
You need to find two numbers,so let the numbers be x and y (you dont have to say which number is which, this is something that you are going to find).
So, now you start collecting the data.
The sum of the two numbers ( x and y as we said earlier) equals 1. So:
x + y = 1 (a)
Also,the difference of the two numbers (again x and y ) equals 0.7. So:
x - y = 0.7 (b)
Now we have collected the data in two statements.The statement (a) and the statement (b).Now, we get to solve the system of these two equations:
(a)→x=1-y
Thus, (b) equals to:
(b)→1 -y -y = 0.7
Going on with (b):
-2y=0.7 - 1→ -2y = - 0.3 → y = 0.15
So, since we have found the number y, we are able to solve one equation of those above ( we can solve (a) or (b) ).
I choose to solve (a).So:
x + 0.15 = 1 → x= 1 - 0.15 → x= 0.85
Now that you see that the numbers are found ( x= 0.85 and y=0.15 ) you can verify the equations (a) and (b) by using their arithmetic values.
