Frank took 4 history tests. Frank earned an 85, 92, and 89 on the first three tests. If Frank's average (arithmetic mean) for all 4 tests is 91, what did he earn on his last test?
解析： Write an equation for the average of the four tests. Use x for the unknown score. . Solve for x 85+92+89+x=364 x=98. Frank earned 98 on the fourth test.
. Solve for x 
85+92+89+x=364