Ball 1, with a mass of 10kg, is moving to the right at 20m/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 original speed. what is the mass of ball 2?

Short answer, 10kg, it must be the same mass as Ball 1

Long calculation to prove the

M1 = Mass of Ball 1
M2 = Mass of Ball 2
U1 = Initial speed of Ball 1
U2 = Initial speed of Ball 2
V1 = Final speed of Ball 1
V2 = Final speed of Ball 2

M1U1 + M2U2 = M1V1 + M2V2

10 x 20 + M2 x 20 = 10 x 40 + M2 x 0

200 + 20M2          = 400

20M2                   = 200

M2                      = 200 / 20

M2                      = 10 kg

Alternatively you could say that because the velocity of ball 1 increased by the exact velocity of ball 2, their masses must be equal, so both ball 1 and 2 are 10 kg.