Pulley puzzle

A special type of chain pulley can be used to manually lift very heavy objects like car engines.

There are two pulleys, red (with radius 5) and blue (with radius 4), that move together and determine the maximum weight that can be lifted. Pulling on the slack part of the chain (black dots) wrapping around the pulleys lifts the box.

What is the mechanical advantage of this pulley? (That is, if I pull with a force of 1 Newton, how many times of upward force is applied on the brown box?)

Assumptions:

  • Assume that the two lines that pull on the brown box are vertical (that is, there is no loss of mechanical advantage due to a bad angle).
  • Assume that the pulleys and the chain are weightless and that there is no friction.
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