which statement is not true about variable?

Right.

To answer the question about which statement is not true about variables, we will evaluate each option:

a) A variable is used to represent a quantity whose values may change or vary - This statement is true. Variables are placeholders for values that can change.

b) Letters are often used as variables - This statement is true. In mathematical expressions, letters are commonly used to represent variables.

c) A variable is a rule showing relationships among quantities - This statement is not true. A variable represents a specific value, not a rule or relationship among quantities.

d) Known values can be substituted for variables in a formula - This statement is true. In mathematical formulas, variables can be replaced with known values.

Therefore, the statement "c) A variable is a rule showing relationships among quantities" is not true.

Moving on to the next question about equivalent expressions for "-8 - (-8)":

a) -8 + 8 - This expression simplifies to 0, which is equivalent to -8 - (-8). So, this is not the correct answer.

b) 0 - This expression simplifies to 0, which is equivalent to -8 - (-8). So, this is not the correct answer.

c) 8 - 8 - This expression simplifies to 0, which is not equivalent to -8 - (-8). Therefore, this is the correct answer.

d) -8 + (-8) - This expression simplifies to -16, which is not equivalent to -8 - (-8). So, this is not the correct answer.

Hence, the correct expression that is not equivalent to -8 - (-8) is "c) 8 - 8".

Moving on to the last question about simplifying the expression "-4 + (-6) - (-4)-(-7)":

a) -21 - This is not the correct answer because the expression simplifies to 1.

b) 21 - This is not the correct answer because the expression simplifies to 1.

c) 1 - This is the correct answer. By simplifying the expression, we get -4 + (-6) + 4 + 7, which equals 1.

d) 0 - This is not the correct answer because the expression simplifies to 1.

Therefore, the correct simplified expression is "c) 1".