standard_description: Solve word problems that call for addition of three whole numbers whose sum is less than or equal to 20, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.
problem_activity: Problem 1
A scientist plants a seed in a laboratory and tracks the growth of the plant. The height of the plant in centimeters is proportional to the number of days since it was planted.
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a.   Explain what point B represents in context of the situation.
b.   At what rate is the plant growing? Indicate this as a point on the graph.
c.   Write an equation that represents the relationship between days and height.
d.   If the plant continues to grow at this rate, how tall will it be after 12 days?
e.   If the plant continues to grow at this rate, when will it be 6 cm tall?
Problem 2
Match each table to its equation.
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i.
$${y = {1 \over 2}x}$$
ii.
$${y = {2 \over 3}x}$$
iii.
$${y = 2x}$$
iv.
$${y=3x}$$
Problem 3
The cost you pay for limes is proportional to the number of limes you buy. Four different stores sell limes for different amounts, as shown in the graphs, table, and equation below. Which store should you go to if you want to pay the least amount for limes? Explain your answer.
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