Bob and his dad visited the hardware store on Saturday. They observed the following transactions:Mr Harris bought 4 for $0.50, Mrs Clarke bought 21 for $1.00, Mr Montaro bought 1,124 for $2.00 and Ms Park bought 450 for $1.50. What were they buying?

Street numbers, at $0.50 each digit.

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To determine what Bob and his dad were buying at the hardware store, we need to analyze the transactions given in the question.

Let's break down each transaction:

1. Mr Harris bought 4 for $0.50:
Here, Mr Harris bought 4 units of an item and paid $0.50 for them.

2. Mrs Clarke bought 21 for $1.00:
Mrs Clarke bought 21 units of the same or a different item and paid $1.00 for them.

3. Mr Montaro bought 1,124 for $2.00:
Mr Montaro bought a significantly larger quantity of 1,124 units for $2.00.

4. Ms Park bought 450 for $1.50:
Ms Park purchased 450 units for $1.50.

Now, looking at the different quantities and prices, it becomes apparent that all the transactions involve different amounts of units and prices. Hence, each transaction most likely refers to a different item being sold at the hardware store.

To determine what Bob and his dad were specifically buying, we may require more information or further context.