Thread: Mathematical gymnastic problem- my daughter's homework!!

I am ashamed to admit that my mind cannot get it!!

Here is the problem. It requires working backward.

In the 1980s the rhinoceros population decreased by 485 from what it was in the 1970s. By the 1990s the population increased to 2 more than twice the population in the 1970s. By the 2000s the population dropped 25 rhinos to about 7 rhinos today. What was the population in 1970s?

Thanks in advance.

70's=x
80's=y
90's=z
00's=t

y=x-485
z=2x+2
z-25=7

We know z=32
therefore we know x=15
which DOESNT MAKE SENSE. There appears to be a problem in the question itself, or my understanding of it.

Originally Posted by mathatlast

...By the 1990s the population increased to 2 more than twice the population in the 1970s..

Are you sure it isn't meant to read:

By the 1990s the population increased to 2 more than twice the population in the 1980s ?

Thank you for the help. I agree that the problem is poor.

Originally Posted by pomp

Are you sure it isn't meant to read:

By the 1990s the population increased to 2 more than twice the population in the 1980s ?

I agree- I think it is a typo- it is for a 5th grader and they have not even learned formal algebra.

Originally Posted by mathatlast

I agree- I think it is a typo- it is for a 5th grader and they have not even learned formal algebra.

If the typo was changed to 1980's then the question is straightforward and the answer is a nice round number of rhinos.

So in the 2000's we've got 7 rhinos and we're told that is 25 less than in the 90's. Therefore in the 90's there were 7+25=32 rhinos.
We're then told there are twice as many plus two rhino's in the 90's as there were in the 80's. Therefore there were (32-2)/2 = 15 rhinos in the 80's.
Then we're told that there are 485 less rhinos in the 80's than 70's, therefore there were 15+485=500 rhinos in the 70's.

I think the point of this question is to break things down into smaller steps and to check for understanding on reversing the operations that have taken place.

I dont remember doing anything this complex in fifth grade. Poor girl!