Algebra is a means to generalizing any math process involving numbers in our number system.
A strong foundation in numbers and operations is essential. Your child should be comfortable writing numbers in their expanded form, know the properties of arithmetic and be able to apply them to integers. They must also be exceptionally comfortable dealing with fractions.
Being able to articulate generalizations from arithmetic and patterns is a skill used throughout algebra.
It's time to start gathering the materials you need to help your child slow down or stop the Math Summer Slide. Start with this resource of daily math facts practice. 
At about Fourth grade, your child should have the ability to solve algebraic equations and inequalities using procedures and algebra facts. They will rely less and less on observations and trial and error.
They must know the following before progressing in Algebra
Properties of Arithmetic 
Exponents

As your child progresses through fifth and sixth grade, they will become more and more comfortable with exponents, radicals and dealing with monomials.
On completing 6th Grade your childs basic algebra knowledge should include the following.
Radicals

Monomials 
Once these skills are mastered the fun really starts. Polynomials are introduced along with graphing equations, solving equations and then the joys of quadratic equations.
This is starting to travel beyond the mathematics on this site, but I do love Algebra, so I might just expand on this topic in the future!
There is nothing Basic about algebra. As you can see here, there are so many rules to learn and master. The good thing however, is that algebraic terms follow the exact rules of arithmetic. You can't add X and Y, just like you couldn't add 1/2 and 1/3!
Algebra is a living subject  it surrounds us. Be sure to make algebra real for your kid  it might just be a subject they love!
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