Cards numbered 1 to 40 are put in a box and one card is drawn at random. What is the probability that the number on it is NEITHER a multiple of 4 NOR a multiple of 10?

Multiples of 4: 10 of them. Multiples of 10: 4 of them. Multiples of both, i.e. of 20: 20 and 40, so 2. Numbers in at least one list = 10 + 4 - 2 = 12, so neither = 40 - 12 = 28 and $P=\frac{28}{40}=\frac{7}{10}.\frac{3}{10}$ answers the opposite event; $\frac{13}{20}$ forgets to subtract the 2 counted twice; $\frac{17}{20}$ ignores the multiples of 10 completely.