There is a mistake in your video lecture
sir in your video lecture you have said that mass number= atomic mass, but if an element has isotopes then the mass number and atomic mass is different and in your video lecture every question of atomic mass there is no symbols amu or u is not mentioned. Explain
If an elements have isotopes, then atomic mass of that element is taken mean value of all atomic mass of isotopes. Number of proton is same, so atomic number is same for the isotopes. And atomic mass do not need any symbol of amu or u. It can be showed by only numbers also.