(a) $$\dfrac {36}{48} = \dfrac {9}{\square}$$
$$36\times \square = 48\times 9$$
$$3\times 3\times 2\times 2\times \square = 2\times 2\times 2\times 2\times 3\times 3\times 3$$
$$\square = 12$$
Hence the required fraction is $$\dfrac {9}{12}$$
(b) $$\dfrac {36}{48} = \dfrac {\square}{4}$$
$$36\times 4 = 48\times \square$$
$$3\times 3\times 2\times 2\times 2\times 2 = 2\times 2\times 2\times 2\times 3\times \square$$
$$3 = \square$$
Hence, the required fraction is $$\dfrac {3}{4}$$.