一包巧克力里有牛奶巧克力、黑巧克力和橙味巧克力。其中 25% 的巧克力是黑巧克力。黑巧克力数量与牛奶巧克力数量的比是 5 : 7。黑巧克力和橙味巧克力一共有 78 块。这包巧克力一共有多少块？

1. Find the percentage of milk chocolates using the given ratio

The ratio of dark to milk chocolates is $5 : 7$. Since dark chocolates make up $25\%$ of the total, $5 \text{ units} = 25\%$.
$1 \text{ unit} = 25\% \div 5 = 5\%$.
Percentage of milk chocolates = $7 \times 5\% = 35\%$.

2. Find the percentage of orange chocolates

Percentage of orange chocolates = $100\% - 25\% - 35\% = 40\%$.

3. Find the total percentage of dark and orange chocolates

Percentage of dark and orange chocolates = $25\% + 40\% = 65\%$.

4. Calculate the total number of chocolates

We are given that there are $78$ pieces of dark and orange chocolates.
$65\% \rightarrow 78$
$1\% \rightarrow \frac{78}{65} = 1.2$
$100\% \rightarrow 1.2 \times 100 = 120$
The total number of chocolates is $120$.