Answer
The eclipsed form has torsional energy = 15 KJ/mol (3.6 kcal/mol) Theta = $0^{\circ}$ and $120^{\circ}$
Staggered Theta = $60^{\circ}$ and $180^{\circ}$
Work Step by Step
C-H eclipsed with C-H contributes to 4.2 kJ/mol (1 kcal/mol) and
C-H eclipsed with C-CH3 contributes to 5.4 KJ/mol (1.3 kcal/mol) to the torsional energy of the conformations.
and since we have one C-H eclipsed with C-H and to C-H eclipsed to C-CH3 then
4.2+5.4+5.4 = 15 KJ/mol
or
1+1.3+1.3 = 3.6 kcal/mol
is the potential energy of the eclipsed conformations.
