calc using Interface

std-3/5_CalculatorUsingInterface/README.md

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+# CalculatorUsingInterface
+## Problem Statement
+
+Writing same contract in different ways increases understanding. 
+
+So, in this problem statement, we will rewrite the Calculator contract using Interface. We will use [this contract from std-2](../../std-2/2_SafeCalculator/SafeCalculator.sol)
+
+Task is to rewrite the same contract using an `interface`.
+
+## Informational Hint
+
+### Interfaces
+
+Interfaces are similar to abstract contracts, but they cannot have any functions implemented. There are further restrictions:
+
+- They cannot inherit from other contracts, but they can inherit from other interfaces.
+
+- All declared functions must be external in the interface, even if they are public in the contract.
+
+- They cannot declare a constructor.
+
+- They cannot declare state variables.
+
+- They cannot declare modifiers.
+
+Know more: https://docs.soliditylang.org/en/v0.8.20/contracts.html#interfaces