Answer:
Cluster sample
Step-by-step explanation:
This is an example of a cluster sample. In a cluster sample, the examiner divides the population into groups (each one of these groups is called a cluster) and once the examiner has these clusters, takes one of them and recollects the data from ALL the members of that cluster. In this case, the teacher divided the class in 3 different groups and then selects one of these groups and asks the average amount of time per week he/she spent studying. 
 
        
             
        
        
        
Answer: it equals 20
Step-by-step explanation:
10 x 20 = 120, hoped this helped.
 
        
                    
             
        
        
        
Answer:
Third answer (she is incorrect because she should have squared each leg length and then found the sum.)
Step-by-step explanation:
The pythagorean theorem states that a²+b²=c². This is not equivalent to (a+b)²=c² (due to FOIL expansion, this expands to a²+2ab+b²=c²).
This matches with the third answer, as she has to do a² and b≥ separately.
**This question involves expanding perfect squares, which you may wish to revise. I'm always happy to help!
 
        
             
        
        
        
Answer:
a and c
Step-by-step explanation:
The volume (V) of a cuboid is 
V = lwh ( l is length, w is width, h is height )
The volume is given as 210 cm³
Check each of the dimensions given to determine which give the correct volume
a
V = 15 × 2 × 7 = 210 ← correct volume
b
V = 14 × 3 × 7 = 294 ← incorrect volume
c
V = 7 × 3 × 10 = 210 ← correct volume
Hence a and c  could be the dimensions of the box
 
        
                    
             
        
        
        
Dividing two fractions is the same as multiplying the first fraction by the reciprocal of the second fraction. remember KFC: keep, change, flip. 
a/b ÷ c/d *use KFC* = a*d/b*c 
=ad/ bc
2/7 ÷ 4/5 = 2/7*5/4
=10/28
(simplify)
=5/14