Heuristic - Guess and Check

Dear all,
I hope that the Guess and Check method you did today is one skill you have mastered.
Let's try the question below on a rough piece of paper ( It has to be a decent piece of paper.)

Mrs Lee bought 60 pens and pencils. A pencil cost $1.20 and a pen cost $1.50. She spent $84 on all the 60 pens and pencils. How many pens and how many pencils did she buy? 

Questions to ask yourself:
What is the guessing number?
What is the checking number?
How to make use of $1.20 and $1.50?
What details to put in my table?

Happy solving....

Love you,
Mrs Mak