2^15 / 2^17 = 2^(15 - 17) = 2^(-2) = 1/2²2 = 1/4
2^7 × 5^13 × 3^20 / 10^9 × 9^11 = 2^7×5^13×3^20 / 2^9×5^9×3^22 =
= 2^(7 - 9)×5^(13 - 9)×3^(20 - 22) = 2^(-2)×5^4×3^(-2) = 1/4 × 1/9 × 5^4 =
= 625/36 = 5^4 / 36
15^13×21^20 / 35^9×99^11 = 3^13 × 5^13 × 3^21 × 7^21 / 5^9 × 7^9 × 11^11 × 3^22 = 3^(13 + 21 - 22) × 5^(13 - 9) × 7^(21 - 9) /11^11 =
3^12 × 5^4 × 7^12 / 11^11