There are 10 boys and 20 girls in a class. The class has a test. The mean mark for all the class is 60. The mean mark for the girls is 54. Work out the mean mark for the boys

Find the total number of people in the class 10+20=30
find the total number of marks achieved by the class 60x30=1800
since the mean mark for the girls is 54, you can work out the total marks achieved by the girls 54*20=1080
you can work out the total marks achieved by the boys by subtracting the total marks achieved by the girls from the total marks achieved by the class 1800-1080=720
to work out the mean mark for the boys divide the total marks echieved by the boys by the number of boys 720/10=72
72 is the mean mark for the boys