Mechanics 1

Hey I have my AS-level mech is a few days and revising i came across this question and i can do A but I can't work out how to do B. Any help would be greatly apreciated. Thank you.

A plank AB has a length 4 m. It lies on a horizontal platform with the end A lying on the platform and the end B projecting over the edge, The edge of the platform is at the point C.

Jack and Jill are experimenting with the plank . jack has a mass of 40 Kg and jill 25KG. THey discover that if jack stands at B and Jill stands at A and BC = 1.6 m, the plank is in equilibrium and on the point of tilting about C By modelling the plank as a uniform rod, and jack and jill as particles

a) find the mass of the plank

They naw alter the position of the plank in relation to the platform so that, when jill stands at B and Jack stands at A, The plank is again in equilibrium and on the point of tilting about C

b) Find teh distace in this position.

yup answer to a is 10.

for b:

40*AC = 10*(2-AC) + 25*(4-AC)
75AC = 120
AC = 1.6m