Je bloque sur l'ex 2 :-\

A = ( 1/3 - 1/5 ) : 2/5
= ( 5/15 - 3/15 ) : 2/5
= 2/15 : 2/5
= 2/15 x 5/2
= 10/30
= 1/3

B = 4/7 - 1/7 x 5/3
= 4/7 - 5/21
= 12/21 - 5/21
= 7/21
= 1/3