Answer:
9
Step-by-step explanation:
Let's say Mark is M and his brother is B.
M+5=B
M+B=23
Now we can solve using subsitution.
M+M+5=23
(Subtract 5 from both sides)
2M=18
M=9
Hope this helped!
Answer:
(a)
Step-by-step explanation:
(a)The degree of a polynomial is the highest power of the unknown variable in the polynomial.
A polynomial is said to be in standard form when it is arranged in descending order/powers of x.
An example of a fourth degree polynomial is: 
We know the polynomial above is in standard form because it is arranged in such a way that the powers of x keeps decreasing.
(b)Polynomials are closed with respect to addition and subtraction. This is as a result of the fact that the powers do not change. Only the coefficients
change. This is illustrated by the two examples below:

The degrees do not change in the above operations. Only the number beside each variable changes. Therefore, the addition and subtraction of polynomials is closed.
Answer:
6²= 6×6= 36
√81= 9
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Answer:
A. Five and two eighths feet.
Step-by-step explanation:
42 (feet of yarn) ÷ 8 (equal parts) = 5.25 (feet of yarn that each part is)
5.25 =

or

because 1/4 = 2/8.
Hope this helps!
Answer:
11
Step-by-step explanation:
11