I'm going through past papers in preparation for my summer exams and I can't for the life of me figure out how to do these three questions...

9. Two pendulums, P and Q, are set up alongside each other. The period of P is 1.90 s andthe period of Q is 1.95 s. How many oscillations are made by pendulum Q between two consecutive instants when P and Q move in phase with each other?
A 19
B 38
C 39
D 78

12. The gravitational field strength at the surface of the Earth is 6 times its value at thesurface of the Moon. The mean density of the Moon is 0.6 times the mean density of the Earth. What is the value of the ratio (radius of the earth) / (radius of the moon) ?
A 1.8
B 3.6
C 6.0
D 10

13. The diagram shows two points, P and Q, at distances r and 2r from the centre of aplanet. The gravitational potential at P is –16 kJ kg–1. What is the work done on a 10 kg mass when it is taken from P to Q?
A – 120 kJ
B – 80 kJ
C + 80 kJ
D + 120 kJ

I know question 9 has something to do with phase difference but I don't know what the equation would be to find the answer...For question 12 would you equate GM/r^2 for the earth and 6GM/r^2 for the moon, and then find M for both using density = m/V? As for question 13 I have no idea how to work that out...

The answers according to the mark scheme are B, B, C respectively, can anyone walk me through how to arrive at the answers? Cheers