Using OwnValues and HoldForm:
{a = 7, b = 5, c = 6}; HoldForm[(a + b)/c] /. OwnValues@a /. OwnValues@b /. OwnValues@c (7 + 5)/6
Using OwnValues and HoldForm:
{a = 7, b = 5, c = 6}; HoldForm[(a + b)/c] /. OwnValues@a /. OwnValues@b /. OwnValues@c (7 + 5)/6