she took four tests with one more to take totals 5 tests
88 x 5 = 440
all her tests need to equal at least 440 to get an 88 average
since she has 85 average after 4, 85*4 = 340, so her 4 tests so far have equaled 340
440-340 = 100
she will need to get a 100 on her 5th test to have an 88 average
