How do you complete the proportion \[\dfrac{4}{5}=\dfrac{x}{15}\]?

Hint: In this problem, we have to complete the given proportion, by solving and finding the value of x. To solve these types of problems, we can just add, subtract or multiply numbers and cancel similar terms to get the value of x. In this problem we can multiply the number 15 on both sides and cancel using the multiplication tables, therefore, we can find the value of x and substitute in the given proportion.

Complete step-by-step solution:
We know that the given proportion is,
 \[\dfrac{4}{5}=\dfrac{x}{15}\] …… (1)
Here, we have an unknown variable x, which we have to solve and find.,
Now, we have to find the value of x.
Now we can multiply the number 15 on both sides in the proportion (1), we get
\[\Rightarrow 15\times \dfrac{4}{5}=\dfrac{x}{15}\times 15\]
Now, we can cancel the similar terms on the right-hand side and cancel using multiplication tables on the left-hand side, we get
\[\begin{align}
  & \Rightarrow 3\times 4=x \\
 & \Rightarrow x=12 \\
\end{align}\]
The value of the unknown variable x is 12.
Therefore, the final proportion is \[\dfrac{4}{5}=\dfrac{12}{15}\].

Note: Students make mistakes while multiplying/dividing similar terms to cancel, solve and get the value of x. To solve these types of problems, we should know how to cancel using the multiplication tables. In this problem, we have been asked to complete the proportion and hence we have to substitute the resulting value in the place of the variable.