Answer:
Make, sum, total
Explanation:
There are many terms in the word list that could be used to describe how Rob had solved the sum in his head. He had made and arranged numbers vertically in his head then sum it and at last he found the total.
Answer below:
personally when im doing mental math, i break it up. so break up 103 into 100 and 3. Then add 100 to 1875= 1975. Easy right? then add 3 and u get 1978!
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