<span>Cards less than 14 : {2,3,4,5,6,7,8,9,10,11,12,13} 12 instances or P(N<14)=12/100=3/25 Perfect squares : {4,9,16,25,36,49,64,81,100} 9 instances or P(perf. sq.)=9/100 Prime number less than 20: {2,3,5,7,11,13,17,19} 8 instances or P(P<20)=8/100=2/25</span>
Let's check the first statement with an example. 2 and 3 are positive numbers and their sum (5) is also positive so his first statement is true.
To check the second statement let's look at the negative numbers -1 and -8 for example. Their sum (-9) is also negative so his second statement is true.
To check the third statement let's look at the numbers 9 and -5. One is positive and one is negative, but their sum (4) is positive, so his third statement is false. However if we look at the numbers 4 and -7, their sum is negative so the third statement is partially false.