Question 105701
Tina wants to realize a mean income of $390 in her six days of operation.  The total amount of income she'll make, divided by the six days of work will give us this mean.  I.e.,

weekly income / days worked = daily mean

We know that Tina will have worked six days when the week is over, so:

weekly income / 6 = daily mean

And we know that she's hoping to realize a mean of $390, so:

weekly income / 6 = $390

Now to make things look a little more like an equation we'll let x represent the weekly income.

x / 6 = 390

Solving for x we multiply both sides of the equation by 6 and get:

x = 2340

Again, x was how much Tina would have to earn over the six day work week, not how much she needed to make on the sixth day.  
We know how much she made during the first five days; let's add them:

200 + 140 + 360 + 400 + 280 = 1380

So if Tina needs to make $2,340 in six days, and she's already made $1,380 in the first five, the difference between the two will be what she needs to make on the sixth day.  I.e., letting i represent the income needed on day six:
i = 2340 - 1380
i = 960

Tina needs to earn $960 on her sixth day of work to have a mean income of $390 for the six days.