Venn diagram problem

How many in only single?

3 Magazines (D, R, I); total number of people is 1000

250 - D, I, not R
40 - I, R, not D
120 - R, not in I
200 - I, not in D
90 - D, R
70 - none

This is a review but I don't understand

Professor told us to get answer = (y+Z+160)+250+90-x+X=40+70=1000

But I don't understand how to get y, z and x?

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...is 1000

250 - D, I, not R
40 - I, R, not D
120 - R, not in I
200 - I, not in D
90 - D, R
70 - none

There will be ...