The element bromine exists in nature as two isotopes: 79br has a mass of 78.9183 u, and 81br has a mass of 80.9163 u. the average atomic mass of bromine is 79.90 u. calculate the relative abundance (as percentages) of the two bromine isotopes.

Asmall (but heavy) particle placed in a glass of water will follow a zigzag motion because the particle will bounce off of the water molecules it meets. this is called brownian motion. a physicist simulates this on a computer, by varying the distance a particle can travel (called the mean free length), on average, before it collides with a water molecule and assigning the change in motion to be one of 8 directions, each with a similar probability. by running the simulated particle (with the same mean free length) many times she determines that it should take 15 seconds, on average, for the particle to fall to the bottom, with a standard deviation of 1.5 seconds. next she lets a real particle fall through a glass of water and finds that it took 18 seconds. what does she conclude, and why?

Me because some of question i don’t understand

Solve: 3x+y=6 4x-2y=18 show all work !