I wanted to point out that you can't solve this equation because the question you gave us isn't properly formatted and isn't true at the moment... Unless they are 3 different problems. Which I have solved below.
Let's start off with the first question.
50 - 55 + 4 = 15
You would first simplify each side to find...
-1 = 15
Which isn't true meaning this question is unsolvable.
Now, moving onto the second question.
30 + 55 - 6t = 7
You again would start off by simplifying each side.
85 - 6t = 7
Then you should add 6t to each side to get rid of the negative 6t.
85 = 7 + 6t
After that, you should subtract 7 from each side so that you have isolated the variable.
78 = 6t
And the you would divide each side by 6 to get the answer for t.
t = 13
Now, the final question. I will be solving for both r and t in this question. Let's start with solving for t first.
-r + 55 - 5t = 20
You would start off by add 5t to each side to begin the isolation of t.
-r + 55 = 20 + 5t
Then you would subtract 20 from each side to completely isolate the variable.
-r + 35 = 5t
Now you would divide 5 from both sides to just get t.
-r/5 + 7 = t
And that's your answer for t.
To get r, you would begin again by trying to isolate the r. You would add r to both sides to try and isolate r.
-r + 55 - 5t = 20
And you would get...
55 - 5t = 20 + r
Now you would subtract 20 from each side to get r alone...
35 - 5t = r
And that's your answer for r.