The total amount of corn starch left is 320 grams.
He gives each student 80 grams of corn starch from a 2-kilogram bucket.
Each students collected 80 grams of corn starch.
Therefore,
1 student = 80 grams of corn starch
His class has a total of 21 students.
Therefore,
total mass of corn starch distributed = 21 × 80
total mass of corn starch distributed = 1680 grams
The corn starch from the bucket = 2 kg
1kg = 1000 g
2kg = 2000g
Hence,
The corn starch from the bucket = 2000 grams
Finally,
amount in grams of corn starch left = 2000 - 1680
amount in grams of corn starch left = 320 grams.
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