For the reaction A + 2B → C ; 5 moles of A and 8 moles of B will produce

The reaction      A    +    2B  $\to$   C

acc. to eqn       1mole  2moles      1mole

Given data       5moles   8moles     ?

1 mole of A reacts 2 moles of B

5 moles of A reacts --------?

10 moles of B reacting but given data 8 moles of B reacts so B is limiting agent

2 moles of B produce 1 mole of C

8 moles of B produce   ------?

$\frac{8\times 1}{2}=4$moles of C