solving a word problem

Question

A textbook store sold a combined total of 399 psychology and chemistry textbooks in a week. The number of psychology textbooks sold was two times the number of chemistry textbooks sold. How many textbooks of each type were sold?

Answer 1

Each sentence gives you an equation. If p is the number of psych books and c is the number of chem books, then:
p + c = 399

The second sentence tells you that:
p=2c

So you can use one equation to substitute in to the other. In this case, the easiest way would be to change the first to:

2c + c = 399 (because p and 2c are the same)

From there:

3c = 399

 

I assume you can take it from there...