40% of 50 students in a class are good at Science. How many students are not good at Science?

Given that, 40% of 50 students in a class are good at Science.

∴ Number of students good at Science

$=40\% \mathrm{of} 50$

$=\frac{40}{100}\times 50$

$=20$

Therefore, the number of students not good at Science = 50 – 20 = 30