On Monday, Brendan gave Tristan twice as many cards Tristan already had. On Tuesday Tristan gave Brendan 1/3 of his cards. On Wednesday Brendan gave Tristan as many cards as Tristan already had after that each had 36 cards. How many cards did Brendan and Tristan each have at the start of Monday.

Let,

Tristan's cards are x unit

Brendan's cards are y unit

Monday,

Tristan has 2x cards

Tuesday,

Tristan has 2x/3 cards

Wednesday,

Tristan has 4x/3 cards

According to question 4x/3=36 ... (1)

Again,

Monday,

Brenden has y-2x cards

Tuesday,

Brenden has y-2x + 2x/3 cards

Wednesday,

Brendan has y-2x + (2x/3) - (2x/3) = y-2x

According to question y-2x=36 ... (2)

From (1),

4x/3=36

or, 4x=108

or, x=27

By putting x=27 in (2),

y-2*27=36

or, y-54=36

or, y=90

So Tristan had 27

and Brendan had 90 cards at beginning of monday ...