5: Get the variables on one side and the constants on the other.

9: This is like #5, except there are more like terms to combine.

11: Start by distributing the 10 and the 2.

12: A negative times a negative is positive.

15: Start by distributing, and then combine like terms.

17: Solve the equation to find the value of h that makes the two expressions equal.

19: An impossible equation like 2 ＝ 3 is never true (no solutions), and an identity like 2 ＝ 2 is always true (infinitely many solutions).

22: See hint for #21.

25: They did not do the same thing to both sides of the equation.

26: Zero always equals zero, no matter what the value of the variable is.

27: Let x be the number of months, and put an expression for Company A on one side of the equation and an expression for Company B on the other.

39: If the variable term is the same on both sides, then when it is subtracted from each side the resulting equation will be always true like 2 ＝ 2 or never true like 2 ＝ 3.

41: The absolute value symbol |. | changes any negative value to positive.