Need help with this math question above

First doing it by mental math.

a. you do the same thing that you do when evaluating the expression but do it in your mind. First add the numbers before the decimal point and then add the numbers after decimal point. Then add the decimal where in the equation the decimal point lies. I got 13.0 or 13 when I did it in my mind.

b. Over we see that we have a negative number in the beginning and then positive number. You must be thinking that this is going to be hard to calculate it in your mind, but actually it is very easy. add 5 + 4.6 + 1 in your mind. You will get 10.6. Now subtract 10.6 - 5.6. You will get 5.

c. This is also easy. You are given a mixed number. What you will do is first solve the whole numbers in your mind (3, 4, 1). 3 - 4 + 1 = 0. Now you are only left with fraction. Solve the fractions in your mind. This is a trick that I use that doesn't work in all fraction. Sometimes I will have to solve it on paper, but sometimes I can just use my mind to solve. If you see in the equation you have 2 fractions with same denominator. So, add 1/6 + 5/6 = 1. Now subtract 1 - 3/4. It will give you 1/4.

Now let us solve it on paper. I have attached a picture of it.