The sum of $1 + 3$ is $4$.

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Here are 5 related questions for further practice:

1. What is $2 + 5$?
2. What is $3 + 7$?
3. What is $4 + 6$?
4. What is $8 + 2$?
5. What is $9 + 3$?

Tip: Addition is commutative, meaning $a + b = b + a$.