So basically everything but choices B and C are functions.
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Explanation:
A. This is a function as there are no repeated input x values. Each input (x) leads to exactly one output (y).
B. This is not a function. We have the input 2 lead to more than one output (1 and 4 at the same time)
C. This is not a function. Similar to choice B, we have multiple outputs for a given input. In this case, x = 20 has more than one output.
D. This is a function. Any x input will produce only one y output. When graphed, it passes the vertical line test.
E. This is a function. This graph passes the vertical line test. It is not possible to draw a vertical line through more than one point on the blue curve, so that's what I mean by "passing the vertical line test".
F. This is a function. At any given hour (x), there is exactly one temperature (y) possible. The thermometer is not going to produce multiple readings at any one specific moment in time.
Chris will solve his problem by dividing because using division can tell you a missing factor, and vice versa. 10 / 10 = 1, so 1 is the missing factor. Then Chris can multiply: 10 X 1 = 10. So, Chris will divide, then multiply.