v is velocity, x is position, t is time, and a is acceleration.
For the same reason that you can skate around a curve at constant speed but not with constant velocity.
The DIRECTION you're going is part of your velocity, but it's not part of your speed.
If the DIRECTION changes, that's a change of velocity.
The object doesn't have to change speed to have a different velocity. A change of direction is enough to do it.
And any change of velocity is called acceleration.
The amount left of a given substance can be calculated through the equation,
A = (A0) x 0.5^n/h
From the given scenario,
A/A0 = 0.75 = 0.5*(60/h)
The value of h from the equation is 144.565 minutes.
90.9 is the velocity of an airplane