The grocery store is a great place to begin exposing your child to math concepts. Experiences can be developed around counting, measurement, unit ratios, algebra, etc. in a very practical way.
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As an example: Pre-Schoolers can practice the skills related to equal, more and less. (Common Core Standard: K.CC.C.6 – Identify whether the number of objects in one group is greater than, less than or equal to the number of objects in another group.)