I do not understand.

can u understand the model?

2/5 of total = 3/5 of remaining

this is the key


Based on the last sentence in the question
total : M : S+J = 5 : 2 : 3
M : J : M+J = 3 : 2 : 5

try to use M to link all ratio
total : M : S+J = 5 : 2 : 3 = 15 : 6 : 9
M : J : M+J = 3 : 2 : 5 = 6 : 4 : 10

S = total - M - J = 15 - 6 - 4 = 5

S : total = 5:15 = 355 : 1065

Ans: 1065【3 times of Singapore stamps】