s =-T2+ 15t-50(5 & lt； t & lt 10)

(2) There are two situations.

(2t- 10)/( 10-t)= 10/20

t=6

20-(t- 10)=30-t

(2t-40-30+t)/ 10= 1/2

t=25

So t=6 or t=25.