What would be an example of the law of conservation of matter

Two friends at different locations want to communicate with each other by sending low energy signals. which of the following methods can they use to communicate? a) produce x-rays using colliding electrons and send them to radios, which capture sound b) send messages using infrared radiation, which travel in the form of waves c) send radio waves through intervening media like radio and television d) produce sound waves using microwaves from heated objects

Flourine is found to undergo 10% radioactivity decay in 366 minutes determine its halflife

If 1.8 l of water is added to 2.5l of a 7.0 molarity koh solution, what is the molarity of the new solution