who can help me thanks Maths problems?

If L = MN, what is the percentage increase in L if M increases by 15% and N increases by 20% ??

hello, im studying out of collegeboard’s SAT test prep book, and i came across a math problem that I don’t understand how to solve… it only provides the answer 

If 18√18 = r√t , where r and t are positive integers and r > t, which of the following could be the value of rt? 

a) 18 

b) 36 

c) 108 

d) 162 

e) 324 

The answer is C but I dont get how… because it all makes sense to do 18 x √18(which is around 4.24) = 76.32 because 18 > √18..?? or 36 x √36 (6) = 216 ?