Answer:
Let's work backwards to figure out how many cookies were left after Mrs. Johnson ate 3 cookies:
Before Mrs. Johnson ate the cookies, the class had eaten 5/6 of the cookies, which means that 1/6 of the cookies were left.
Let's call the number of cookies in 1/6 of the batch "x". So the total number of cookies is 6x.
After the class ate 5/6 of the cookies, there were 1/6 of the cookies left, which is x.
Then Mrs. Johnson ate 3/4 of the cookies that were left, which means she ate 3/4*x cookies.
We know that Mrs. Johnson ate 3 cookies, so we can set up the equation: 3 = 3/4*x
Solving for x:
3 = 3/4x
34/3 = x
4 = x
So there were 4 cookies in 1/6 of the batch, which means there were 6 times as many, or 6*4 = 24 cookies in the whole batch.
Therefore, Mrs. Johnson brought 24 cookies to class that day.