A force P of 50 N and another force Q of unknown magnitude act at 90° to each other. They are balanced by a force of 130 N. The magnitude of Q is A. 60 N B. 80 N C. 100 N D. 120 N

The correct answer is $\boxed{\text{B) 80 N}}$.

To solve this problem, we can use the Pythagorean theorem. The Pythagorean theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. In this case, the hypotenuse is the force of 130 N, and the other two sides are the forces P and Q. So, we have the equation:

$130^2 = P^2 + Q^2$

$16900 = 2500 + Q^2$

$Q^2 = 14400$

$Q = \sqrt{14400}$

$Q = 80$

Therefore, the magnitude of Q is 80 N.

Option A is incorrect because 60 N is not a solution to the equation $130^2 = P^2 + Q^2$.

Option C is incorrect because 100 N is not a solution to the equation $130^2 = P^2 + Q^2$.

Option D is incorrect because 120 N is not a solution to the equation $130^2 = P^2 + Q^2$.