To find the answer to this equation we can start by rearranging it.
8 + j = 15
Could be turned into...
15 - 8 = j
We switch the equation to make it easier to solve.
Now we solve the equation.
15 - 8 = 7
So our answer is j = 7.
If you would like to check the answer, simply plug in what we got for j. Like shown...
8 + j = 15
8 + 7 = 15
And now we know that j = 7 is 100% correct.