Hey sunshine,

Pls find my worked solution as follows:

<Before>

B : R

3 : 1

W - 1 whole

<Process>

Sold (2/5) B, none R, (1/3) W

<After>

B : R

9/5 : 1 -- (x5)

W - (2/3)

(splitting 1p into 5u each)

B : R

9 : 5

Hence do likewise for <Before> ratio.

<Before>

B : R

15: 5

W - 1 whole

Given that there were 2180 (B,R,W) T-shirts at first, and 1504 (B,R,W) left, we have ...

20u + 1W --> 2180 --- (1)

14u + (2/3)W --> 1504 --- (2)

Eliminating W:

(2)x(3/2):

21u + 1W --> 2256 --- (3)

Comparing (1),(3):

(3)-(1):

21u - 20u --> 2256 - 2180

1u --> 76

Number of R T-shirts at first = 5u --> 5 x 76

= 380

================================

Trust this helps.

Do let me know again if there's any clarification.

Cheers,

Edward