First, rewrite the expression as:
#(16/28)(p^2/p)(r^3/r^5) =>#
#((4 xx 4)/(4 xx 7))(p^2/p)(r^3/r^5) =>#
#((color(red)(cancel(color(black)(4))) xx 4)/(color(red)(cancel(color(black)(4))) xx 7))(p^2/p)(r^3/r^5) =>#
#4/7(p^2/p)(r^3/r^5)#
Next, use these rules of exponents to simplify the #p# terms:
#a = a^color(blue)(1)# and #x^color(red)(a)/x^color(blue)(b) = x^(color(red)(a)-color(blue)(b))# and #a^color(red)(1) = a#
#4/7(p^color(red)(2)/p^color(blue)(1))(r^3/r^5) => 4/7(p^(color(red)(2)-color(blue)(1)))(r^3/r^5) => 4/7(p^color(red)(1))(r^3/r^5) => #
#4/7(p)(r^3/r^5) => (4p)/7(r^3/r^5)#
We can now use this rule of exponents to simplify the #r# terms:
#x^color(red)(a)/x^color(blue)(b) = 1/x^(color(blue)(b)-color(red)(a))#
#(4p)/7(r^color(red)(3)/r^color(blue)(5)) => (4p)/7(1/r^(color(blue)(5)-color(red)(3))) => (4p)/7(1/r^2) => #
#(4p)/(7r^2)#