A frictionless pendulum of length 3 m swings

Question

with an amplitude of 10◦. At its maximum
displacement, the potential energy of the pendulum is 10 J.
What is the kinetic energy of the pendulum
when its potential energy is 5 J?
1. K = 10 J
2. K = 3.3 J
3. K = 6.7 J
4. K = 15 J
5. K = 5 J

Answer 1

5. K =5 J

since the only force acting on it is gravity, it's a conservative force. That means the energy is only transferred from U to KE and never lost.
We use the formula:
E= U + KE
At its max displacement, KE=0 and E must equal U. It says that at its max displacement U=10J so at that point E also equals 10J.
So if E=10 and U=5, KE=?
E= U+KE
10=5+KE
KE=5J

Answer 2

5 J