In order to return to the question you were working on, you would need to navigate back to the Practice section that it was under (i.e., if the topic name is "Equations with variables on both sides: decimals & fractions," then navigate back to that topic on Khan Academy and look for "Practice." When you begin the assessment once again, it should return you back to the question you were working on. I can also add some insight on the mathematics if you are unable to return to the practice:

We start with...

2/3b + 5 = 20 - b

from here, as you noted, you can add b to both sides of the equation to yield:

5/3b + 5 = 20

You already had 2/3b, and you are adding b. Note that adding b is equivalent to adding 1 * b because you are adding "one b." 1 is equivalent to 3/3 (any number divided by itself is equivalent to 1), so 1 * b is equivalent to 3/3 * b. Thus, 2/3b + 3/3b = (2/3 + 3/3)b = (5/3)b. Next, you can subtract 5 from both sides of the equation:

5/3b = 15

Then you can divide by (5/3). 

b = 15/(5/3)

Dividing by a fraction is the same thing as multiplying by its reciprocal, so...

15/(5/3) is equivalent to 15 * (3/5), which is 45/5 or 9.

Therefore, you have b = 9.

Hi Antonio Carlos,

My name is Carol and I'm from the Khan Academy support team!

I am afraid that there isn't a way to go back to a question and there isn't a reset button for exercises at the moment.

why not? why isnt there a way to back to a question? isnt reviewing questions we have done a key way of learning?