How strong are broken bones after they heal compared to before break?

Just as strong. Fractured bones, in an otherwise healthy, well-nourished person should heal uneventfully if the fracture is reduced and immobilized adequately for enough time to allow healing. The strength of the healed bone should be essentially equal to the strength before the injury occurred.
Strong!! It depends, but once a bone is healed usually it is as strong or stronger than before you broke it. There are many other factors that go into answering the question, like was surgery necessary to fix the bone and how was it fixed, how old you were when you broke the bone, etc...