Each part: 1 Mark
a) XXX. 
Now, we now that if a symbol is repeated, its value is added as many times as it occurs.
XXX = 10 + 10 + 10 = 30.
b) IX
I=1
X=10
1<10
If a smaller value is written to the left of a symbol of greater value, its value is subtracted from the value of the greater symbol.
$\therefore$ The value IX = 10 - 1 = 9.
c) VII
If a symbol of smaller value is written to the right of a symbol of greater value, its value gets added to the value of the greater symbol.
V=5
I=1
$\therefore$ The value of VII = 5 + 1 + 1 = 7.
d) XII
In this case, also, the smaller value is written to the right of the greater number. So its value is added to the value of the greater symbol.
XII= 10 + 1 + 1 = 12.