Question

The sum of 10 times a number and 15 is added to eleven times the same number

Answer 1

10x+15+11x=21x+15
Hope this helps :)

Answer 2

If you would like to translate the phrase into the math expression, you can do this using the following steps:

the number ... n
the sum of 10 times a number and 15 ... 10 * n + 15
eleven times the same number ... 11 * n

Now, we have to add this two parts of the phrase:
10 * n + 15 + 11 * n = 21 * n + 15

The correct result would be 21 * n + 15.