Answer:the pages will come on the first is 210
Step-by-step explanation:
Since we have given that two pages that face each other in a book 
i.e. they must be consecutive to each other 
so, let the first page number be x
and the second page number be x+1
According to question, 

2x=421 -1
2x=420

So, the pages will come on the first is 210