Answer:
7.
Step-by-step explanation:
21 = 3 * 7
28 = 2 * 2 * 7
42 = 2 * 3 * 7
Collecting common factors:
t is common to all 3 sets of factors so
HCF = 7.
The fundamental theorem of calculus tells you

He earned 200 more dollars the 6th year than the 5th year
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