Let's first simplify 48/144 ALL the way, and then simplify 5/15 ALL the way. Then, from there, if both fractions simplified are the same, then they are equal to one another.
48/144 : Start with small number ; divide both numerator and denominator by 4.
12/36 : Divide by 3.
4/12 : Divide by 4.
1/3 = 48/144
Now, simplify 5/15
5/15 : Divide by 5.
1/3 = 5/15
So, because both 48/144 and 5/15 equal 1/3, these fractions must be equal to each other.