**Example 1** - mental math

Since the equation has infinitely many solutions, the coefficient of x in the left expression must be 9 and the constant in the right expression must be 6. Hence, k = 3 and c = 2.

**Example 4** - mental math

It can be observed that the constant in left and right expression is same. For the equation to have no solution, 0.5a cannot be 10. Hence, a cannot be 20.