They use the logarithm with base 10 which is standard. If the base wasn't 10 the autors would have indicated that for sure. And it's not useful to write the base 10:
Q: Given that Lg x = p and lg y = q, express the following in terms of p and q.
a) xy
Ans ￼: = 10^p x 10^q
= 10^(p+q)
Where the 10 come from?! Like lg10, but if it is it will be xlg10 which is x(1). I don't get this. Please help.