1. A geologist has 10 rocks of equal weight
A geologist has 10 rocks of equal weight. If 6 rocks and a 10-ounce weight balance on a scale with 4 rocks and a 22-ounce weight, what is the weight of one of these rocks in ounces?
Difficulty:
Medium 4 5 6 7 8
HINT
If r is the weight of one rock, then the weight of 6 rocks and a 10-ounce weight can be written as 6r + 10, and the weight of 4 rocks and a 22-ounce weight can be written as 4r Since the two sides balance, these weights are equal.

2. C: 6
Explanation
Let r equal the weight of each rock, in ounces. The weight on one side of the scale is 6r + 10 ounces and the weight on the other side is 4r + 22 ounces. Since the two weights balance, you can write the equation 6r + 10 = 4r Solving this equation yields r = 6.