First you need to know the definition of a complementary angle. That means that two angles add up to 90 degrees, or a right angle.
To find an angle complementary to a 78 degree angle, you need to subtract 78 from 90.
90 - 78 = 12
The decimal in standard form for 12 and six hundredths is 12.06

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Write the divide fraction horizontally:
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Factorise the numerators and denominators when possible:
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Convert the divide fraction to multiplication fraction
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Cancel the factors
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Combine to single fraction
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Answer: 36
Since the sum of the ratios is 5+7=12
the total number of students must be divisible by 12
The only number within the given range is 36