Danny Completed 40% of his homework assignment his assignment contain 30 math problems how many problems does Danny’s still need to complete

Step-by-step explanation:

Well, he completed 12 questions, so he has 18 left.

To get the solution, we are looking for, we need to point out what we know:  

We assume, that the number 30 is 100% - because it's the output value of the task.  

We assume, that x is the value we are looking for.  

If 30 is 100%, so we can write it down as 30=100%.  

We know, that x is 40% of the output value, so we can write it down as x=40%.  

Now we have two equations:

1) 30=100%

2) x=40%

where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:

30/x=100%/40%

Now we just have to solve the equation.

30/x=100/40

(30/x)*x=(100/40)*x       - we multiply both sides of the equation by x

30=2.5*x       - we divide both sides of the equation by (2.5) to get x

30/2.5=x  

12=x  

x=12

now we have:  

40% of 30=12

Then, you need to subtract 12 from 30 to figure out what is left.