Question

the molar mass of a substance is 20 grams then the molecular mass of the substance

Answer 1

The molecular mass of that substance
$= 20 \: u$

Answer 2

if the given substance is a molecular substance them molar mass is nothing but the molecular mass of the substance

the mass of 1 mole of substance is nothing but molar mass

in case of atoms the gram atomic mass is the molar mass

in case of molecules gram molecular mass and molar mass