Print PDF worksheet above, the answers are on the second page.
Algebraic expressions are required in the curriculum as early as the 7th grade, however, the foundations to performing the tas occurs in the 6th grade. Thinking algebraically occurs with using language of the unknown and representing the unknown with a letter. When presenting a question like: The difference between a number and 25 is 42. Difference should signify that subtraction is implied and knowing that, the statement would then look like: n - 24 = 42. With practice, it become second nature!
I had a teacher that once said to me, remember the rule of 7 and re-visit. He felt if you performed seven worksheets and re-visited the concept, you could claim that you would be at the point of understanding. So far it seems to have worked.