The reaction is:
2NH3 + H2SO4 -----> (NH4)2SO4
it tells us that 2 moles of ammonia yields 1 mol of Ammonium sulfate.
So we just need to state that:
2 moles NH3 - 1 mol (NH4)2SO4
30 moles NH3 - x
x = (30) (1) / 2 = 15 moles of (NH4)2SO4
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