Andy has 6u marbles.

Ben has an unknown quantity B.

Scenario 1

Andy gives Ben ¹/₃ -> 2u

Andy has 4u remaining.

Ben has B + 2u

(B + 2u) – 4u = 20

B – 2u = 20

Scenario 2

Andy gives Ben ½ -> 3u

Andy has 3u remaining.

Ben has B + 3u

(B + 3u) – 3u = 50

B  = 50

Taking (B – 2u) = 20, and B = 50,

50 – 2u = 20

2u = 30

u = 15

Andy has 6u -> 90 marbles