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A and B react together in this reversible reaction.

A + 3B ⇌ C + 2D

A mixture of 10 mol of A and 10 mol of B were left to reach equilibrium. The equilibrium mixture contained 4 mol of B.

What is the total amount, in moles, of substances in the equilibrium mixture?

A) 14
B) 16
C) 18
D) 20

If B has 4 moles remaining at eqbm, that means six miles reacted. 1:3 ratio with A and B respectively therefore A only has 6/3 moles that react which = 2, leaving 10-2= 8 moles at eqbm. 2 moles of C formed and 2*2=4 moles of D formed.
8+4+2+4= 18, therefore I think it is C.