Answer:
96
Explanation:
1) lets take "x" as the 6th test score
so to find average we need to take sum of all numbers divided by number of numbers:
(88+85+89+92+90+x)/6=90
so
88+85+89+92+90=444
(444+x)/6=90
444+x=540
x=96
Gillian needs a 96 on her next test