3.87 - 2.59 = 3 + 0.87 - 2 + 0.59

=$ \left(3 +\dfrac{87}{99} \right) $-$ \left(2 +\dfrac{59}{99} \right) $

= 1 +$ \left(\dfrac{87}{99} -\dfrac{59}{99} \right) $

= 1 +$ \dfrac{28}{99} $

= 1.28.