[rosellaxoxo]

A mixture of Mg, Al, Al2O3 of 12.3g reacted with NaOH. After the reaction, there are 5mol of H2 and 24g of leftover chemicals. Find the masses of the products.
Mg + NaOH  No Reaction
Al + NaOH + H₂O  NaAlO₂ + H₂
Al₂O₃ + NaOH  NaAlO₂ + H₂O
 
(sorry for any language mistake. I translated the exercise from another language.)

[mikasaur]

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[AWK]

You need balanced reactions.
Note - your data are unsolvable. 12.3 g of this mixture or 12.3 g NaOH (it is not clear for me) never give you 5 moles of H₂. For 123 g of the mixture this is possible but at the assumption that Al2O3 does not react.
Moreover,
for dissolution of Al₂O₃ you need an elevated temperature. Then also Mg reacts with hot water.