He is making the step out of concrete. The concrete must be made up of \[ 1 \text { part cement: } 2 \text { parts water: } 3 \text { parts shingle. } \] He uses a small bucket to measure each part. He estimates that he will need 18 buckets of concrete in total. How many buckets of shingle will he need?

To solve this problem, we first need to determine the total number of parts in the concrete mixture. According to the given ratio, the concrete is made up of 1 part cement, 2 parts water, and 3 parts shingle. This means there are a total of 1 + 2 + 3 = 6 parts in the mixture. Now, we know that he estimates he will need 18 buckets of concrete in total. Since the mixture is made up of 6 parts, each bucket of concrete will contain 18 buckets / 6 parts = 3 parts of the mixture. Since the shingle is 3 parts of the mixture, and each bucket contains 3 parts, he will need 3 parts of shingle per bucket. Therefore, for 18 buckets of concrete, he will need 18 buckets * 3 parts of shingle per bucket = 54 buckets of shingle.