Chemistry Problem - Pearson (Higher Level) Ch. 1

The combustion of both ammonia, NH3, and hydrazine, N2H4, in oxygen gives nitrogen and water only. When a mixture of ammonia and hydrazine is burned in pure oxygen, the volumetric N2 : H2O ratio in the product gas is 0.40. Calculate the % by mass of ammonia in the original mixture. What assumptions are being made here?

I don’t quite understand how you’d solve this problem. Can someone explain the steps please?