For

and

, it's helpful to remember the half-angle identities:


So


For

, the Pythagorean identity suffices:

which means

since

.
X - speed of the current
40+x - speed with current
40-x - speed against current
t-time
(40+x)*t=100
(40-x)*t=60
40t+xt=100
<span> + 40t-xt = 60
</span> 80t =160
t = 2
40t+xt=100,
40*2+2x=100
80+2x=100
2x=100-80
2x=20
x=10 mi/h
10 mi/h speed of the current.
come to your house to pick it lowest to go with D