The number of seventh-graders that must join the club in order to meet the president's wishes is 10.
Since there are 45 eighth graders and 20 seventh graders in a school club and the president of this club wants 40% of the club’s members to be seventh graders, we need to find the number of seventh graders that will be added to the school club to make their percentage 40 %.
Let the number of seventh-graders to be added be x.
So, the new number of seventh-graders is 20 + x and the new number of people in the club is 45 + 20 + x = 65 + x
Thus, the percentage of seventh graders in the club is thus
[tex]\frac{20 + x}{65 + x} X 100[/tex] %
Since this percentage equals 40 %, we have that
[tex]\frac{20 + x}{65 + x} X 100[/tex]% = 40 %
[tex]\frac{20 + x}{65 + x} = \frac{40}{100} \\\frac{20 + x}{65 + x} = 0.4[/tex]
Cross-multiplying we have
[tex]20 + x = 0.4(65 + x)[/tex]
Expanding the bracket, we have
[tex]20 + x = 26 + 0.4x[/tex]
Subtracting 20 from both sides we have
[tex]x = 26 - 20 + 0.4x\\x = 6 + 0.4x[/tex]
Subtracting 0.4x from both sides, we have
[tex]x - 0.4x = 6\\0.6x = 6[/tex]
dividing both sides by 0.6, we have
x = 6/0.6
x = 10
So, the number of seventh-graders that must join the club in order to meet the president's wishes is 10.
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