a cyclist went 1km up a hill at 15km per hour then she went

a cyclist went 1km up a hill at 15km per hour then she went down the hill at 30km per hour show that her average speed for the 2km was 20km per hour

D=RT
2=20T
T=2/20
T=.1 HOUR.
1=15*X
1=30(.1-X)
15X=30(.1-X)
15X=3-30X
15X+30X=3
45X=3
X=3/45
X=.0666 ANS FOR THE TIME GOING UP THE MOUNTAIN.
.1-.0666=.0334 ANS. FOR THE MINUTES DOWN THE MOUNTAIN.
PROOF:
15*.0666=30(.1-.0666)
1=30*.0334
1=1