To solve for a, it is important to remember that we can only add or subtract like terms. Also, that we have to isolate the variable should be solved to one side and all others to the other side. We do as follows:
1/4(20−4a)=6−a
5 - a = 6 -a
- a + a = 6 - 5
0 = 1 ------------> OPTION C. NO SOLUTION
