18. A group of students took a test which consisted of 3 questions. We know that $90 \%$ answered the first question correctly, $80 \%$ answered the second question correctly and $70 \%$ answered the third question correctly. What is the smallest possible percentage of students who answered all three questions correctly?
A $30 \%$
B $35 \%$
C $40 \%$
D $50 \%$
E $70 \%$
SolUTION
C

There are a group of $10 \%$ who failed to answer correctly the first question, $20 \%$ for the second and $30 \%$ for the third.

The number who failed on one or more questions will be largest if these groups do not overlap; and in that case they will form $60 \%$ of the whole group. So the smallest possible percentage of students who got all correct is $40 \%$.