School has 15 sixth graders 12 sevenths and 13 eight graders what percentage of math club members are either sixth or seventh graders?

Assuming the math club had 15 sixth graders, 12 seventh graders, 13 eighth graders and no other members, then:

There are 15 + 12 = 27 students that are sixth or seventh graders.

There are 15 + 12 + 13 = 40 members.

So the fraction that are sixth or seventh graders is 27/40.

Using long division, 27/40 = 0.675 = 67.5%

So 67.5% of the math club members are sixth or seventh graders.