First, group like terms and rewrite the expression as:
#((8 * 3)/(15 * 2))(a/a)((b * b^-1)/b^4) =>#
#((8 * 3)/(15 * 2)) * 1 * ((b * b^-1)/b^4) =>#
#((8 * 3)/(15 * 2))((b * b^-1)/b^4) =>#
#((color(red)(cancel(color(black)(8)))4 * color(blue)(cancel(color(black)(3))))/(color(blue)(cancel(color(black)(15)))5 * color(red)(cancel(color(black)(2)))))((b * b^-1)/b^4) =>#
#4/5((b * b^-1)/b^4)#
Next, use these rules for exponents to multiply and simplify the #b# terms in the numerator:
#a = a^color(red)(1)# and #x^color(red)(a) xx x^color(blue)(b) = x^(color(red)(a) + color(blue)(b))# and #a^color(red)(0) = 1#
#4/5((b * b^-1)/b^4) => 4/5((b^color(red)(1) * b^color(blue)(-1))/b^4) => 4/5(b^(color(red)(1)+color(blue)(-1))/b^4) =>#
#4/5(b^color(red)(0)/b^4) => 4/5(1/b^4) =>#
#4/(5b^4)#
