A school has 800 students, and 320 of them participate in the science club. What percent of the students participate in the science club?

To find a percent, divide the part by the whole and multiply by 100. Here the part is 320 science club members and the whole is 800 students: $\frac{320}{800} = 0.40$, which is 40%, choice C. Check: 10% of 800 is 80, and $4 \times 80 = 320$. Choice A (32%) comes from reading the 320 as if it were 32 per hundred, or from moving the decimal point incorrectly. Choice B (80%) uses the complement, since 480 students are not in the club, which is 60%, not 80%. Choice D (25%) would be 200 students. Always divide by the total group size.