Forces and equilibrium — practice question

A box with mass $8\text{ kg}$ rests on a rough plane that is tilted at $5^\circ$ to the horizontal. A force of size $P\text{ N}$ is applied to the box, directed upwards and parallel to the line of greatest slope of the plane. For $P = 7X$, the box travels up the line of greatest slope with acceleration $0.15\text{ m s}^{-2}$, whereas for $P = 8X$, it travels up the line of greatest slope with acceleration $1.15\text{ m s}^{-2}$.