The half-life of a substance is 10 years. What percent of the substance remains after 2 half-lives. need answer asap. i think its 2.5 but not sure

On mars a rock falls an unknown vertical distance from a resting position and lands in a crater. if it takes the rock 2.5 seconds to fall, how high is the cliff the rock fell from? mars' surface gravity is 3.8 m/s2.

Bonds within one water molecule are examples of

What is a transverse wave? how do the particles in the medium move in relation to the energy of the wave? does this wave require a medium?