Answer:
10 g
Explanation:
Right from the start, just by inspecting the values given, you can say that the answer will be  
10 g
.
Now, here's what that is the case.
As you know, a substance's specific heat tells you how much heat is needed to increase the temperature of  
1 g
 of that substance by  
1
∘
C
.
Water has a specific heat of approximately  
4.18
J
g
∘
C
. This tells you that in order to increase the temperature of  
1 g
 of water by  
1
∘
C
, you need to provide  
4.18 J
 of heat.
Now, how much heat would be required to increase the temperature of  
1 g
 of water by  
10
∘
C
?
Well, you'd need  
4.18 J
 to increase it by  
1
∘
C
, another  
4.18 J
 to increase it by another  
1
∘
C
, and so on. This means that you'd need
4.18 J
×
10
=
41.8 J
to increase the temperature of  
1 g
 of water by  
10
∘
C
.
Now look at the value given to you. If you need  
41.8 J
 to increase the temperature of  
1 g
 of water by  
10
∘
C
, what mass of water would require  
10
 times as much heat to increase its temperature by  
10
∘
C
?
1 g
×
10
=
10 g
And that's your answer.
Mathematically, you can calculate this by using the equation
q
=
m
⋅
c
⋅
Δ
T
  
, where
q
 - heat absorbed/lost
m
 - the mass of the sample
c
 - the specific heat of the substance
Δ
T
 - the change in temperature, defined as final temperature minus initial temperature
Plug in your values to get
418
J
=
m
⋅
4.18
J
g
∘
C
⋅
(
20
−
10
)
∘
C
m
=
418
4.18
⋅
10
=
10 g