Solve Decimal Addition: 5.1 + 2.3 Step by Step

To solve this problem, let's follow these steps for decimal addition:

• Step 1: Align the decimals of the numbers vertically, so that both $5.1$ and $2.3$ have their decimal points in the same position:

   5.1
+  2.3
---------

• Step 2: Start adding from the rightmost column (the tenths position). Add the tenths: $1 + 3 = 4$.
• Step 3: Move to the left to add the units: $5 + 2 = 7$.

Put these sums together, and we have:

   5.1
+  2.3
---------
   7.4

Therefore, the sum of $5.1$ and $2.3$ is $7.4$. This matches choice $\text{3}$.

Thus, the correct answer is $7.4$.