=lg20/lg2-lg25/lg4
=lg20/lg2-lg5²/lg2²
=lg20/lg2-2lg5/2lg2
=lg20/lg2-lg5/lg2
=(lg20-lg5)/lg2
=lg(20/5)/lg2
=lg4/lg2
=lg2²/lg2
=2lg2/lg2
=2