Let's see: meal * tax will give you the number of tax dollars to put on your bill, and meal + that number will give you the cost of the meal + tax!
The interpreter will evaluate the right-hand side of the equation first. It will ask itself, "What's meal * tax?" Once it does this and gets the result (about 3.00), it will ask itself, "What's meal + this number?" (44.50 + about 3.00 = about 47.50.) Finally, it'll move to the left-hand side of the equation and say, "Oh man, I should totally reassign meal (which was 44.50) to this new value of about 47.50." And that's how meal gets reassigned!