Operator sign is not clear to me. I assume it is "-" for both operators.
If it is a "-" sign, then
10^8 - 10^2 / 10^7 - 10^3
10^(8-2) / 10^(7-3)
10^6 / 10^4
10^(6-4)
10^2
700+ approximation
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papgust, I think you're using a rule we use when dividing, not when subtracting 
Notice here that 10^8 is 100 million, while 10^2 is just one hundred; 10^8 is one million times larger than 10^2. So 10^8 - 10^2 is only a tiny amount less than 10^8. Similarly 10^7 absolutely dwarfs 10^3 in size; 10^7 - 10^3 is only very slightly less than 10^7. So (10^8 - 10^2)/(10^7 - 10^3) is very close in value to 10^8/10^7 = 10.
Notice here that 10^8 is 100 million, while 10^2 is just one hundred; 10^8 is one million times larger than 10^2. So 10^8 - 10^2 is only a tiny amount less than 10^8. Similarly 10^7 absolutely dwarfs 10^3 in size; 10^7 - 10^3 is only very slightly less than 10^7. So (10^8 - 10^2)/(10^7 - 10^3) is very close in value to 10^8/10^7 = 10.
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