Answer
Answer choice $A$: $\boxed{30+10a}$
Work Step by Step
If $a$ is the number of pairs that wrap around a blue bar, $3-a$ is how many pairs that wrap around the red bar. You get 20 points for each pair that wraps around the blue bar, and 10 points for each pair that wraps around the red bar. This is the expression for how many points you get:
$$20a + 10(3-a) = 20a + 30 - 10a = \boxed{30 + 10a}$$
