On the last three physics exams a student scored 86,89 , and 91. What score must the student earn on the next exam to have an average of at least 90 ?

Step 1 ：The student wants to have an average of at least 90 after four exams. The average is calculated by summing all the scores and dividing by the number of scores. In this case, we know the sum of the first three scores and we want to find the fourth score that will make the average at least 90.

Step 2 ：We can set up the following equation to represent this situation: \(86 + 89 + 91 + x \geq 90 * 4\)

Step 3 ：We can solve this equation for x to find the minimum score the student needs on the next exam.

Step 4 ：\(x = 360 - (86 + 89 + 91)\)

Step 5 ：\(x = 360 - 266\)

Step 6 ：\(x = 94\)

Step 7 ：Final Answer: The student must score at least \(\boxed{94}\) on the next exam to have an average of at least 90.