Word Problems Algebra

Mia has some mangoes. Her mom gives her 5 more. Now, she has 12 mangoes. How many mangoes did Mia have at first?

Leo had some toy cars. He gave 3 to his friend and now has 7 left. How many toy cars did Leo have before?

A box contains x pencils. Mia buys another pack of 8 pencils. Now she has 20 pencils in total. How many pencils were in the box?