Sq ft of bldg provided, feet of bldg burnt given, questions ask to represent the burnt structure in a fraction  please help

Hey guys i take a fire dept exam here in a few days and there are a few math questions i cannot figure out how to compute, well i figured out a few but my method doesnt work for some of the other questions leaving me stuck finding a formula or something that works for them all...thank you and here we go

Instructions: all calculations must be done in head no scratch paper, i have 10 seconds to answer each question, speed is paramount, "some of your answers will not be exact and may be rounded, ex- if the answer is close to 1/3 but not exaclty 1/3 choose the closest choice in the answer key"

i will provide 4-5 questions now
questions- "if a building is 4085 square feet and 1600 feet of the bulding burns what fraction of the bldg is left?" answer-3/5

if a building is 2825 square feet and 2100 feet of the bulding burns what fraction of the bldg is left?--ans--1/4
if a building is 1600 square feet and 700 feet of the bulding burns what fraction of the bldg is left?--ans--1/2
if a building is 1010 square feet and 650 feet of the bulding burns what fraction of the bldg is left?----ans----4/10
if a building is 2425 square feet and 300bfeet of the bulding burns what fraction of the bldg is left?-----answer--7/8

please help me figure out a way to successfully complete this math in my head within the time frame, i dont need EXACT I just need to be close enough to the right fraction to choose the right answer on the multiple choice.... my exam in sunday im overall prepared however this portion is what is snagging--keep in mind this must be done in my head within 10 seconds with no paper so the simplier the better, and as long as i am close enough to choose the right fraction from mltpl choice.. thank you!

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