Answer
Least score required on the fourth test: $187$
Work Step by Step
Maximum score I can get: $3\times100+200=500$
To get an A:
$90$% of $500$
$0.9\times500=450$
My scores on the first three tests: $87,92,84$
Total: $87+92+84=263$
Least score required on the fourth test:
$450-263=187$
