Find the edge of the static-friction interval

A block of weight 100 N rests on an incline with sinθ = 0.6 and cosθ = 0.8. The coefficient of static friction is 0.5. A controllable force F acts parallel to the slope upward.

What is the minimum upward F that can hold the block at rest?

Organize the givens & assumptions

Weight = 100 N
sinθ = 0.6; cosθ = 0.8
μs = 0.5

Rigid incline, dry static friction with |f| ≤ μsN, no other forces; the holding force is parallel to the incline so it does not change N.

A $20\ \mathrm N$
B $60\ \mathrm N$
C $100\ \mathrm N$
D $40\ \mathrm N$

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