Math Strategy: Trial & Error - Addition / Subtraction

Math Strategy: Trial & Error - Addition / Subtraction
Math Strand: Number sense and numeration

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Overview

Trial and Error is a strategy that can be used when students are solving addition situations where the whole and one part are known, and one part is unknown. For example; Jack has some lollipops. Susie gives Jack 8 more lollipops. Now Jack has 14 lollipops. How many lollipops did Jack start with? The whole (14 lollipops) is known and one part (Susie gave Jack 8) is known. The student may use trial and error to find what the missing part is. The student adjusts their second guess based on what the outcome of their first guess is.







How this supports student learning

A student using trial and error to solve the lollipop problem will begin with the known part (8) and add another part to try to find the known total (14). The "guess" is not random but anchored to what they already know about number. For example, the student may reason that because they know that 8 + 8 = 16, the missing part could be 7. They will either use a concrete manipulative or counting on/all strategy to determine that 8 + 7 is 15 and is too high. They will then adjust their next guess based on what they discovered. The student is developing their understanding of part-whole relationships as they adjust the missing part to find the known whole.




Knowing the answer is too high, the student will adjust their answer and try 8 + 7





The student will again adjust their answer, based on what they discovered during the last trial.



Where to next
Whole group activities like "Mystery Number" are another way to practice the trial and error strategy.

An example of Mystery Number:

I have 7 candies and I get some more. Now I have 20. How more did I get?

Working as a whole group students can use familiar facts to help make reasonable predictions. For example, they make realize that 7 is 3 away from 10, so the mystery number is a group of 10 and a group of 3, or 13. Modelling this using concrete materials is a great way for students to visually see when they are over or under.

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