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 and explanation:
Discrete random variables can be counted as integers and you can't divide them. Continuous random variables are counted as real numbers and are magnitudes that can´t be counted as an exact number (there will always be an uncertainty in the measurement).
a. The random variable is continuous. Distance is a measurement.
b. The random variable is discrete. You can count how many people are sitting at a computer.
c. The random variable is continuous. Weight is a measurement.
d. The random variable is discrete. You can count how many fishes were caught.
e. The random variable is discrete. You can count how many hints a web site had.
Step-by-step explanation:
Subtracting equation (2) from equation (1)
Substituting d = - 3 in equation (1), we find:
Hence, first term is zero.
Answer:
122
Step-by-step explanation:
x = 180-(180-50-72) = 180-(130 - 72) = 180 - 130 + 72 = 50 + 72 = 122
-4 = y + 3
y = -4x - 3
Solve for Y in the first equation:
-4 = y +3
Subtract 3 from both sides:
y = -7
Now using the addition method:
rewrite the second equation as 4x+y = -3
Multiply y = -7 by - 1 to get -y = 7
Now add the two together:
4x +y = -3
-y =7
4x = 4
Divide each side by 4:
x = 4/4 = 1
Then replace x in one of the equations to solve for y which is -7
X = 1, Y = -7