Although I replied late, nobody gave the answer.
The answer you are looking for is C). But how do you know it's correct? Do some calculation.
Child bike: 4 hours to build + 4 hours to test = 8 hours total
Adult bike: 6 hours to build + 4 hours to test = 10 hours total
5*4 = 20 build hours for child bikes
5*4 = 20 test hours for child bikes
15*6 = 90 build hours for adult bikes
15*4 = 60 test hours for adult bikes
20 + 20 = 40 total hours for child bikes
90 + 60 = 150 total hours for adult bikes
20 + 60 = 80 total test hours
20 + 90 = 110 build hours
Since this amount is below the maximum, the bike order DOES meet the restrictions.
Our equations should be 4c + 6a ≤ 120 for the total building hours, and 4c + 4a ≤ 100 for the total testing hours.
In conclusion, the answer is C) Yes, because the bike order meets the restrictions of 4c + 6a ≤ 120 and 4c + 4a ≤ 100.