苏太太烤了一些芝士玛芬和覆盆子玛芬。她卖掉了 $\frac{3}{5}$ 的芝士玛芬和 $\frac{2}{5}$ 的覆盆子玛芬。剩下的玛芬中，有 $\tfrac{8}{11}$ 是芝士玛芬。苏太太一共卖掉了 760 个玛芬。

1. Find the ratio of the number of cheese muffins left to raspberry muffins left.

Since $\frac{8}{11}$ of the muffins left were cheese muffins, the remaining $\frac{3}{11}$ were raspberry muffins.
Ratio of cheese muffins left to raspberry muffins left = $8 : 3$.

2. Express the total number of each type of muffin in units.

Fraction of cheese muffins left = $1 - \frac{3}{5} = \frac{2}{5}$.
Fraction of raspberry muffins left = $1 - \frac{2}{5} = \frac{3}{5}$.
Let $\frac{2}{5}$ of cheese muffins be $8$ units. Then $\frac{1}{5}$ of cheese muffins is $4$ units, so total cheese muffins = $5 \times 4 = 20$ units.
Let $\frac{3}{5}$ of raspberry muffins be $3$ units. Then $\frac{1}{5}$ of raspberry muffins is $1$ unit, so total raspberry muffins = $5 \times 1 = 5$ units.

3. Calculate the total number of muffins sold and the total baked in units, then find the fraction sold.

Total muffins baked = $20 + 5 = 25$ units.
Cheese muffins sold = $\frac{3}{5} \times 20 = 12$ units.
Raspberry muffins sold = $\frac{2}{5} \times 5 = 2$ units.
Total muffins sold = $12 + 2 = 14$ units.
Fraction of muffins sold = $\frac{14}{25}$.