A is 3 years younger than C but one year older than D . D is one...

According to the question,

$A = C - 3 . . . . . . . . . . ( i )$

$A = D + 1 . . . . . . . . . . ( ii )$

$D = B + 1 . . . . . . . . . . ( iii )$

$D = C - 4 . . . . . . . . . . ( iv )$

$C = 15 . . . . . . . . . . . . . . . . ( v )$

Substituting~the value of C in equation~$( iv )$, we get,

$D = 11$

Substituting~the value of D~in equation~$( iii )$, we get,
$B = 10$

$∴$~B's age is $10$ years.&