What is the value of x in the equation x – y = 30, when y = 15?

Y= 15

x-y  = 30
x-15= 30
    x = 30+15
    x = 45

Therefore, the value of x is 45

X - y = 30
x = ? when y = 15
Substitute for y:- 
x - y = 30
x - 15 = 30
x ⇒ ?
Inverse operation:-
30 + 15 = x
30 + 15 = 45
x ⇒ 45

Check:-
x - y = 30
y ⇒ 15
x - 15 = 30
x ⇒ 45
45 - 15 = 30
Correct!

x ⇒ 45

C. i believe it is (5×(-20))+(100×(0.7))