Consider the example of a^2, this is a^1xxa^1=a^2=a^(1+1)
So a^5, if so chosen, could be written as a^(2+3)=a^2xxa^3
color(blue)("Back to your question")
color(brown)("Method 1")
Write 15^4" as "15^4 xx1
Write 15^6" as "15^(4+2) = 15^4xx15^2
So 15^4/15^6=(15^4xx1)/(15^4xx15^2)
This is the same as 15^4/15^4xx1/15^2
But 15^4/15^4=1 giving
color(brown)(15^4/15^6=1/15^2 = 1/225)
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
color(brown)("Method 2")
Consider the example: 1/a^2 = a^(-2)
Write 15^4/15^6" as "15^4xx15^(-6)
color(brown)(=15^(4-6)=15^(-2) = 1/15^2 =1/225)