Babu made a $4\%$ loss by selling $12$ toffees for $\text{Rs. }5$. How many toffees should he sell for $\text{Rs. }10$ to get a $28\%$ profit?

Let the cost price of $12$ toffees be $\text{CP}$.
Since there is a $4\%$ loss when selling for $\text{Rs. }5$:

$$0.96\times \text{CP}=5⟹\text{CP}=\frac{5}{0.96}=\frac{500}{96}=\text{Rs. }\frac{125}{24}$$

So, the cost price of $12$ toffees is $\text{Rs. }\frac{125}{24}$.
The cost price per toffee is:

$${\text{CP}}_{\text{one}}=\frac{125}{24\times 12}=\text{Rs. }\frac{125}{288}$$

To make a $28\%$ profit, the selling price per toffee (${\text{SP}}_{\text{one}}$) must be:

$${\text{SP}}_{\text{one}}={\text{CP}}_{\text{one}}\times 1.28=\frac{125}{288}\times \frac{128}{100}=\frac{5}{9\times 1}=\text{Rs. }\frac{5}{9}$$

Let $n$ be the number of toffees sold for $\text{Rs. }10$ at this rate:

$$n\times {\text{SP}}_{\text{one}}=10⟹n\times \frac{5}{9}=10⟹n=\frac{10\times 9}{5}=18$$

Thus, he must sell $18$ toffees.