Ball 1, with a mass of 10kg, is moving to the right at 20 m/s. ball 2 is moving to the left at 20 m/s. upon collision, ball 2 comes to a standstill, and ball 1 moves to the left at twice its origional speed. what is the mass of ball 2? .

For us to know the mass of ball 2, we will be using the law of momentum conservation which is the momentum before collision and the momentum after collision (m1v1+m2v2=m1v1+m2v2):

(10×20) + (m×−20) = (10×−40) + (m×0)

200 − 20m = −400

Since we are looking for “m” which is the mass of ball 2, derivation will be needed.
Then:

m = (−400−200) / (−20) = 30kg

Therefore, it is concluded that the mas for ball 2 is 30kg.

Yes

Ohm's law tells you that i = v/r where v is equivalent voltage and r is equivalent resistance of the circuit
since all circuits have the same voltage, your question is really asking which circuit has the lowest total resistance, since the lower the resistance, the higher the current
circuit a has a resistance of 5 ohms; circuit c is in series so has a resistance of 10 ohms; circuit b is in parallel and therefore has a resistance of 2.5 ohms (review your notes on adding resistances in parallel and series); therefore b has the highest current