21) We need to find the tension in rope 3 in the figure above. Many people are tricked into thinking the 10kg block and the tension in rope 1 is relevant. However, they are not. Let's look at the force diagrams to see it.
We can see that there is tension from rope 2 and 3 and gravitational force acting on the 30kg block. Since rope 2 and 3 seem to be identical and point in the same direction, we should assume that the tension forces produced from both ropes are equal in magnitude. We can find the sum of forces and try to solve for the tension in rope 3.
Answer: B
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