As sushi88 mentioned, the phrasing of the question is ambiguous.
In my answer below, I am assuming it is:
"On the second day, he spent 1/4 of his remaining money...".


The amount left of $6.40 is equal to 1 small blue unit.
So at the end of Day 1, we have 8 blue units = 8 x 6.4 = $51.20.
Add $4 to get 3/4 of starting amount = $55.20
So amount of money he had at first is 55.20 * 4/3 = $73.60.

@mr.tan,  it has to be one of those typo errors that we have stated, either a missing word or a number typed wrongly.  *high-five*

However, I won’t say the question is ambiguous.  It is erroneous.   Ambiguous means possibly more than one answer.   Erroneous means the question actually cannot be solved at all.  lol   Sorry I am also particular about English.  lol

This question cannot be solved because the numbers are incorrect.

Say, let original amount of money be 4u

Total  money   [       u           ][       u           ][       u           ][       u           ]

                           [    Spent      ][ 2 bx tea     ][ 2 bxs tea   ][  2 bxs tea  ][1bx tea][$4 misplaced][6.40] 

If 2 boxes tea = u,  5 boxes = 2.5u

So there is not enough money for the 5th box of tea and the $4 misplaced, how to have $6.40 left over?

Question has “Money not enough” …  it cannot be solved.     

However, if I change 3rd day buy 3 boxes of tea...then it is possible.

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Solution after the change:

Total  money   [       u           ][       u           ][       u           ][        u           ]

                           [    Spent      ][ 2 bx tea     ][ 2 bxs tea   ][tea][$4][6.4] 

2 boxes tea —> u   => 1 box =  u/2 = 0.5u

u = 0.5u + 4+ 6.4

0.5u = 10.4

u = 20.8

Hence total money = 4u = 4(20.8) = $83.20