The sum of 6 and a number is 15

15+6=21

Step-by-step explanation:

21

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In mathematics, sum can be defined as the result or answer we get on adding two or more numbers or terms. Here, for example, addends 8 and 5 add up to make the sum 13.

Step-by-step explanation:

https://www.wyzant.com/resources/answers/515691/six_times_the_sum_of_a_number_and_15_is_42

https://www.splashlearn.com/math-vocabulary/addition/sum

for ex: the reciprocal of 3/4 is 4/3. it is the opposite.

reciprocal definition:

a mathematical expression or function so related to another that their product is one; the quantity obtained by dividing the number one by a given quantity.