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You are given that the original distribution of sweets was 3:4:5 between A, B and C.
It means, in translation to human language, that A, B and C possessed 3x, 4x and 5x sweets, respectively,
where x is the common measure of these quantities, now unknown.
But, in addition, you know from the condition that 4x = 48 sweets that B had.
Therefore, x = 48/4 = 12.
Hence, A, B and C had initially 3x = 3*12 = 36 sweets, 4x = 4*12 = 48 sweets and 5*x = 5*12 = 60 sweets, respectively.
Half of the problem is just solved. Now we should analyse the second half.
After C shared his sweets with the brothers, the proportion became 12:16:15.
It means that now A, B and C possess 12y, 16y, and 15y of sweets, respectively.
Here y is the new common measure of their possessions, now unknown.
But we know that A still has 36 sweets, from the first half of the solution.
Hence, y = 36/12 = 3. It implies that after sharing C possess 15*y = 15*3 = 45 sweets.
Hence, 60-45 = 15 is the number of sweets C passed to his brothers. ANSWER
Solved.