20.1.24:Each December 31 in Country Q, a tally is made of the country’s total available coalsupplies

confusedandenthused

I don’t get this problem. Please tell me where my logic flawed.
JY's explanation is here FYI, http://7sage.com/lsat_explanations/lsat-20-section-1-question-24/ and yet I still don't get this.

So according to the explanation,
X—Y= Z
X= the amount that’s been mined.
Y= the amount consumed
Z= the remainder , that is what was mined but not consumed

I plugged in some random numbers and I couldn’t get the logic to follow satisfactorily.

So if we say the Z is 1990 is 50 and the Z in 1991 is 30, then couldn’t the following scenario be possible?
X in ’90= 70
Y in ’90= 20
Z in ’90= 50

Let’s say
X in ‘91= 200
Y in ’91= 170
Z in ‘91= 30

It doesn’t seem 170 is greater than 200. My set up must be wrong.
What I am missing here? Is there something in the "Furthermore, Country Q has not imported or exported coal since 1970" that I am not picking up on? Thanks in advance.

inactive

Bumping this since it was marked as spam for some reason and most people might've missed it.

DumbHollywoodActor

2 words: rollover minutes. You can’t forget to add the surplus from 1990 to 1991.

Mined: X in ’90= 70
Consumed:Y in ’90= 20
Z in ’90= 50

Let’s say
Mined: X in ‘91= 50 (from ’90) + 150 (from ’91)= 200
Consumed: Y in ’91= 170
Z in ‘91= 30

In ’91, the 150 mined is less than the 170 consumed.