Reduce the fraction -20t5 u2 v3 /48t7 u4 v

Your expression, as typewritten, is a bit ambiguous.  Did you intend for 48t7 u4 v to be the denominator?  If so, please enclose  48t7 u4 v inside parentheses.

Then your -20t5 u2 v3 /48t7 u4 v

becomes
 -20t5 u2 v3
 which can be separated for easier reduction:
(48t7 u4 v)

(-20/48) (t^5 / t^7) (u^2 / u^4) (v^3 / v)

Reduce the quantity within each set of parentheses:

(-5/12) (1/t^2) (1/u^2) (v^2)

Please note:  You must use " ^ " to denote exponentation.  u5 is ambiguous; you mean u^5 = "u to the fifth power."