Section 1/#24: The speaker assumes that there is complete overlap between those who would like to join the party and those who would be willing to donate to it. In other words, if all 16 percent who were willing to donate to the party were among the 26 percent who would like to join it, the total percentage in support would be 26. To illustrate the validity of choice E, try some simple numbers:
26 would join (in total)
16 would donate (in total)
5 would both join and donate
Subtracting the 5 people who overlap both categories from the total of both categories (42), we have 37 people in total who would support the party. In summary, choice E points out that the 30-percent requirement could still be met with the percentages given.
Section 4/#17: You're reversing the chain of causation. Here's how it looks according to the stimulus:
large amount of rainfall (April/May) --> increase in the mosquito population --> increased risk of encephalitis
Thus, although a large amount of rainfall may be sufficient (note that the stimulus uses the qualifier "typically") to increase the risk of encephalitis, it isn't necessary for the general threat of encephalitis.