C. They did homework for 2/3 hour
1 1/2 hours can be rewritten as 1.5 hours, 90 minutes, and/or as an improper fraction with a denominator of 6.
First, the whole number (1) as a fraction:
6/6 = 1
Next, we want to rewrite one half (1/2) as a fraction with denominator 6. Since 2 times 3 equals 6, multiply both the numerator and denominator by 3:
1(3) / 2(3) = 3/6.
Lastly, add them: 3/6 + 6/6 = 9/6.
9/6 is equal to 1.5 so this works!!
You are given 5/6 hour which represents the time working on the board. And we found that the class time is a total of 9/6 hours.
Both values have a common denominator so we can preform the operation.
Subtract the values in the numerators and we find the answer:
9/6 - 5/6 = 4/6.
4/6 is also equal to 40 minutes.
4/6 can be simplified as both 4 and 6 are divisible by 2.
Simplified: 2/3
C. They did homework for 2/3 hour