[tex]a)a+(b+c)=(a+b)+c= 3/\frac{3}{5} + \frac{4}{15} = \frac{9+4}{15} = \frac{13}{15} \\ b)(x+y)+z=x+y+z=x+(y+z)= 2/\frac{3}{7} + \frac{17}{14} = \frac{6+17}{14} = \frac{23}{14} \\ c)x+(y+z)+t=x+y+z+t= \frac{17}{47}+ \frac{30}{47}= \frac{47}{47}=1 [/tex]
