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

Careful Anonymous; you've gone wrong in the third line. 

This should be:

x = 30 + 15

so x = 45

.

hi,

this problem is a simple substitution.

so you know that y = 15, so substitute it into the equation you were given!

$${\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{y}} = {\mathtt{30}}$$

$${\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{15}} = {\mathtt{30}}$$

so now with 15 substituted in, all you have to do is move the 15 over to the other side!

$${\mathtt{x}} = {\mathtt{30}}{\mathtt{\,-\,}}{\mathtt{15}}$$

and now simplify by subtracting 15 from 30!

$${\mathtt{x}} = {\mathtt{15}}$$

hope this helps!