Maria throws two stones from the top of the edge of a building with a speed of 20m/s. She throws one straight down and the other straight up. The first one hits the street in a time t1 . How much later is it before the second stone hits?

Aresistor with an unknown resistance is connected in parallel to a 9 ω resistor. when both resistors are connected in parallel to an emf source of 21 v, the current through the unknown resistor is measured with an ammeter to be 4 a. what is the resistance of the unknown resistor? answer in units of ω.