Answer:
780
Step-by-step explanation:
So when you're adding consecutive numbers like this. 1+2+3...+39. You add 39 and 1. This is 40. 2+38 is 40. This is a pattern. Now, we do 40 multiplied by 39 and this will give you the number 1620. After this you would have to divide this product by 2. The answer will soon be 780.
Set it up algebraically
3 times a number so 3x where x = the mystery number
times 7 so 3x*7
and it equals 63 so,
3x*7=63
this can be further simplified to
21x=63
then just divide both side by 21 to get your answer
(21/21)x=(63/21)
so x=3
Answer: the answer is x+8= 1