Explanation
You know that the mean of his 5 scores must be at least 87.
Multiply 5 and 87 to get the total of his scores for the 5 tests (the total needed to get a mean of 87).
\(5 \times 87=435\)
Add the 4 scores that you know.
82+85+87+92 = 346 (total of 4 scores)
Subtract to find the unknown score.
435 – 346 = 89