#549
2) 2n/5m^3 * 10m^5/1 = 4m^2n
4) 72a^4/25b^5 * -5b^4/27a^5 = 8/-15a
#551
1) (-18x^2y^2/5abm) : (-9xy^3/5a^2m^4) = 2xm^3/by
2) 35m^2n/1 : 7m^3/24 = 35m^2n/1 * 24/7m^3 = 120n/m
3) 13a/12xy^2 * 4x^2y/1 = 13ax/3y
5) 6ax^2/1 : 3x^2/4a = 6ax^2/1 * 4a/3x^2 = 8a^2
6) x^2y^2/11ab^2 : 4xy^3/33ab = x^2y^2/11ab^2 * 33ab/4xy^3 = -3x/4by