Answer:
#5a: After 3 weeks, they will have a total score of 415 points.
#5b: (Full work below) Kris' score: 520 ; Justin's score: 450
Step-by-step explanation:

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#5b FULL WORK BELOW
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x=number of weeks: 10
y= total score
Use your equations.

Kris will have 520 points.

Justin will have 450 points.