Qual è la somma di # 2/5 # e # 2/4 #?

First note that #2/4" may be simplified"#

#cancel(2)^1/cancel(4)^2=1/2#

hence #2/5+1/2" is now the sum"#

Since the denominators ( 5 and 2 ) are different we cannot add them. We have to have a common denominator before we can do this.
The lowest common denominator for 2 and 5 is 10. We now express both fractions with a denominator of 10.

#(2/5xx2/2)+(1/2xx5/5)=4/10+5/10#

Now that the denominators are the same we just add the numerators, leaving the denominator(do not add)

#rArr2/5+1/2=4/10+5/10=9/10#