Item 31 forty students are in the science club. of those, 45% are girls. this percent increases to 56% after new girls join the club. how many new girls join?

40 represents a total, but then 45% represents a part. First of all, let's find the amount of girls in the club.

45% of 40 
= 18 girls

If it was increased to 56%:
56% of 40
= estimated 23, since we can't have two fifths of a person. 

23 - 18 = 5 new girls joined.