Let's set it up this way. Let "p" be the price of the calculator. Then we can write this equation. 

p+(6% times p)=$137.80 Rewritten it looks like this:

p+.o6p=137.8 Combine like terms to get:

1.06p=137.8 Divide both sides by 1.06 to get:

p=130

Now we know the calculator cost $130 but check it out for yourself to see when you multiply 130 time 6%. You should get what the tax is, right?