Answer: (to how would I solve this)
"Sales tax is 5%" means multiply $6.00 by 5 and divide by 100 to get tax, and add tax to $6.00.
The "total cost [to] download a book plus tax" is $6.00 + $(6.00 × 5 ÷ 100, round to nearest penny) = $6.30
Step-by-step explanation:
You have to ignore the fact that is some places, digital downloads are not subject to sales tax, and you have to assume that "total cost plus tax" actually means "price plus tax", total amount due including price and sales tax but not, say, your internet access costs, or your monthly prime membership fee. (That last point is actually something to think about). Strictly speaking, total cost would include _all_ costs, and if you want to really irritate someone, you could claim "total cost plus tax" means to include the tax twice. Or, you could claim that "total cost to _download_ plus tax" means you don't include the purchase price but do include tax and internet access fees. (Sorry, sometimes these questions get boring)
be the amount of salt in the tank at time 
. We're given that 
. The rate at which this amount changes is given by
