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

There are two bc a studen that equals to 2