If in a rock collection 60% of the rocks are Quartz how many rocks are Quartz if there are 35 rocks?

If 60% of the rocks are Quartz, you need to multiply the total number of rocks by 60% (this is the same as multiplying by 0.6 as 0.6=60%)

So 35 x 0.6 = 21

Therefore there are 21 quartz rocks in the collection.