Question

why does the mass of tin increase when tin is heated in air

I need a plain blatant answer

Answer 1

The mass of the actual tin will remain constant; however I assume you are comparing the mass of the reactant, tin, with the mass of the products. The increase in mass stems from the oxides of tin present due to its reaction with oxygen in the air:
Sn(s) + O2(g) → SnO2(s)
By working out the atomic mass of the tin and the relative formula mass of tin oxide, you can see why there is a mass increase; 119:151. As you can see here, for every 119g of tin reacted, there will be 151g of tin oxide, hence why you think the mass of the tin increased (by 32g).
Hope this helped!
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Answer 2

because when tin is heated in air then air causes the formation of tinoxide over there.