A clock moves past you at a speed of 0.9 c. How much time passes for you for each second that elapses on the moving clock?

Time dilation formula is

T = T₀ / √ 1-v²/c²

T₀ is time elapsed in moving reference, T time elapsed in stationary reference.

Here T₀ = 1 second

T = 1/√ 1-0.9² = 1/.4358 = 2.3 second

So 2.3 second will pass for each second on moving reference.