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Working Backwards:

In the 1980's the Northen White Rhinoceros population decreased by 485 from what it was in the 1970s. By the 1990s the population increased 2 more than twice the population in the 1970s By the 2000s the population dropped 25 rhinoceroes to about 7 today what was the population in the 1970s

call the population in the seventies X
then in the eighties it was (X-485)
in the nineties it was 2+2X +(X-485) {I am taking the English here literally that the population "INCREASED" an amount 2+2X over what it was in the eighties. I hope you do not have a typo and it should be "INCREASED TO"}
in 2000s 2+2X +X - 485 -25 = 7
so there is your equation to solve for X

500

(7+25-2)/2+485=500

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