Cost Accounting methods(Problem with FIFO)

Hello Sir, I have a problem with the following question Vare produces various inks at its Normanton factory. Production details for Process 1 are as follows: Opening work-in progress, 1 April 400 units 60% complete Closing work-in-progress,30 April 600 units 20% complete Units started 1,000 Units finished 800 The degree of completion quoted relates to labour and overhead costs.Three-quarters of the material are added at the start of the process and the remaining quarter added when the process is 50% complete.The company uses the FIFO method of cost allocation What are the equivalent units of production for materials in the period? A)1250 B)1000 C)850 D)680 Sir my answer was D but the Textbook said C This is how I got my answer 60% was already completed for opening work in progress so 40% needs to be completed 40%*400=160 Units start to finish =Units started-closing work in progress =1000-600 =400 Closing work in progress =600*20% =120 Therefore equivalent units=160+400+120 My answer =680 Sir please correct where I went wrong and how to get the answer for this I also don't understand what to do with the information about "three quarters of the material are added at the start of the process and the remaining quarter added when the process is 50% complete" Overall this is my doubt sir I hope you reply to this and explain me how to do this question Thank You