What single decimal multiplier would you use to decrease by 7% followed by a 20% decrease

Aclassmate thinks that solving a system by graphing gives an exact answer when the lines appear to cross at a grid point, but only an approximate answer when they don't. explain why this isn't true.

At noon a tank contained 10cm water. after several hours it contained 7cm of water. what is the percent decrease of water in the tank?

Masons cell phone company charges $0.12 for every text message he sends plus $2.50 per month if mason sent 25 messages this month how much money does he owe the one with the correct answer and explanation gets 10 points and is marked as the brainliest answer