a) (5^2)^3 / (25^2 * 5^3) = 5^6 / (5^2)^2 * 5^3 = 5^6 / (5^4 * 5^3) = 5^6 / 5^7 = 5^(-1) = 1/5.
1/5 : 1/125 = 1/5 * 125 = 25.