Try to get a rough estimate of the answer as a check on your working. This principle applies to everything in arithmetic, and will often draw attention to some obvious error such as a misplaced decimal. To get an estimate, simply ‘round off’ the numbers involved, e.g. adding 139 and 41.3 is roughly the sum of 140 and 41, so the answer is about 181.
Add both up and down, and look for ‘ten groups’, i.e. when two or more numbers, not too far apart, add up to 10, couple them together, and register them mentally as 10, e.g. should be mentally registered (going from right to left) as 10, 20, 21.
Be sure to place units, tens, hundreds, etc., in their correct position, with the units, etc., exactly under each other.
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