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 = 15You would first simplify each side to find...-1 = 15Which isn't true meaning this question is unsolvable.Now, moving onto the second question.30 + 55 - 6t = 7You again would start off by simplifying each side.85 - 6t = 7Then you should add 6t to each side to get rid of the negative 6t.85 = 7 + 6tAfter that, you should subtract 7 from each side so that you have isolated the variable.78 = 6tAnd the you would divide each side by 6 to get the answer for t.t = 13Now, 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 = 20You would start off by add 5t to each side to begin the isolation of t.-r + 55 = 20 + 5tThen you would subtract 20 from each side to completely isolate the variable.-r + 35 = 5tNow you would divide 5 from both sides to just get t.-r/5 + 7 = tAnd 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 = 20And you would get...55 - 5t = 20 + rNow you would subtract 20 from each side to get r alone...35 - 5t = rAnd that's your answer for r.