Answer:
21
Image result for the sum of 6 and a number is 15
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
