Answer:
Step-by-step explanation:
You can split the coins into 3 groups, each of them has 3 coins. Weigh group 1 vs group 2, if one is lighter, that group has the fake coin. If both groups weigh the same, then group 3 has the fake coin.
Continue to split the group that has the fake coin into 3 groups, each group has 1 coin. Now apply the same procedure and we can identify the fake coin.
Total of scale usage is 2
b) if you have 
coins then you can apply the same approach and find the fake coin with just n steps. By splitting up to 3 groups each step, after each step you should be able to narrow down your suspected coin by 3 times.
Step 1: you narrow down to group of 
coins
Step 2: you narrow down to group of 
coins
Step 3: you narrow down to group of 
coins
...
Step n: Step 1: you narrow down to group of 
coin
You multiply fractions simply by multiplying numerators and denominators with each other:
1.) Which variable did you plot on the x-axis, and which variable did you plot on the y-axis?
x-axis represents people's arms span, y-axis represents their corresponding height
2.) Explain why you assigned the variables in that way.
because you can find the height of people(y) by plugging arm span in x. And i can find people's arm span(x) by plugging height as y
