Let
be the 20 marks of the boys, and
be the 10 marks of the girls.
We know that the global mean was 70, meaning that

Multiplying both sides by 30 we deduce that the sum of the scores of the whole classroom is

By the same logic, we work with the marks of the boys alone: we know the average:

And we deduce the sum of the marks for the boys:

Which implies that the sum of the marks of the girls is 
And finally, the mean for the girls alone is

Answer:
a=15
Step-by-step explanation:
Solve for a by simplifying both sides of the equation, then isolating the variable.
Answer:
.5
Step-by-step explanation:
1 half of any thing is 50 percent and 50 percent is written as a decimal when you move the decimal point twice to the left giving you .5
Answer:
MOP=33
Step-by-step explanation:
MOP + MON = NOP
Substitute what we know
MOP = 3x-3
MON = 4x+9
NOP = 90 right angle
3x-3+4x+9 = 90
Combine like terms
7x+6 =90
Subtract 6 from each side
7x+6-6=90-6
7x = 84
Divide each side by 7
7x/7 = 84/7
x =12
We need to find MOP
MOP = 3x-3
=3(12)-3
36-3
33