How many comic books had Dan?
Writing an equation
We want to know how many books had Dan originally. Let's call this number X.
If Dan sold half of his comic books then, he had after that the other half. That is half of X. That is:
X/2
Then he had 6 more, then he added 6 to his previous number of comic books. That is:
X/2 + 6
He now has X/2 + 6. He now has 17. Then
X/2 + 6 = 17
Finding x
Step 1
Rearranging the terms so the terms with the unkown variable X are on the left side and the other ones on the right side:
X/2 + 6 = 17
↓ substracting 6 both sides
X/2 + 6 - 6 = - 6 + 17
X/2 + 0 = 11
X/2 = 11
Step 2
Solving the equation for X:
X/2 = 11
↓ multiplying by 2 both sides
2X/2 = 2 · 11
X = 22
Answer: he had 22 books. (If he sold half of it, then he had 11, if he bought 6 more, then he had 11 + 6 = 17)