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jmc

number theory junior

Problem

Two different prime numbers between and are chosen. When their sum is subtracted from their product, which of the following numbers could be obtained?
Solution
There are five prime numbers between and namely and Hence the product of any two of these is odd and the sum is even. Because increases as either or increases (since both and are bigger than ), the answer must be an odd number that is no smaller than and no larger than The only possibility among the options is and indeed
Final answer
119