We have a question: There are 25 different layers of rock in a canyon’s strata. Five of the layers are limestone. Half of the rest are sandstone. Four of the remaining are quartz. Half of the rest are shale. The rest are all different kinds of rock. How many kinds of rock make up only one layer in the canyon’s strata?
And an answer: 3
So how you got the answer?
There are 25 - 5 = 20 layers of rock that are not limestone. Half of those 20 are sandstone, so there are 20 / 2 = 10 that are not limestone or sandstone. Four of those are quartz, so 10 - 4 = 6 layers are not limestone, sandstone, or quartz. Half of the rest are shale, so 6 / 2 = 3 kinds of rock remain that make up only one layer in the strata.

We have a question: Amanda needs 12 more marbles to have twice as many marbles as Katrina, and Mabel has 5 times as many marbles as Katrina. If Mabel has 85 marbles, how many more marbles does Mabel have than Amanda?
And an answer: 63
So how you got the answer?
Katrina has 85 / 5 = 17 marbles. Twice as many marbles as Katrina is 17 x 2 = 34. Amanda has 34 - 12 = 22 marbles. Mabel has 85 - 22 = 63 more marbles than Amanda.

We have a question: Jason is mixing a batch of black paint. He needs to add 2 grams of charcoal for every 30 ml of water. If he adds 900 ml of water, how much charcoal does he add?
And an answer: 60
So how you got the answer?
First find how many ml of water Jason needs for 1 gram of charcoal: 30 ml / 2 grams = 15 ml / g. Then divide the amount of water Jason uses by the amount of water per gram of charcoal to find how much charcoal he uses: 900 ml / 15 ml / g = 60 g.