Awesome! Learn SQL interactively on Khan Academy
What is pre-algebra? If you want to be a smart-aleck, I guess that any form of arithmetic before algebra could be considered "pre-algebra." But Khan Academ
Example: two-step equation with numerator x | Linear equations | Algebra I | Khan Academ - YouTube
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Smarthistory on Hagia Sophia, Constantinople (Istanbul). Anthemius of Tralles and Isidorus of Miletus. 532–537 C.E. Brick and ceramic elements with stone and mosaic veneer.
Probability and the Monty Hall problem | Dependent events | Khan Academy
Investment vehicles, insurance, and retirement | Finance and capital markets | Khan Academy
Monty Hall Problem, via YouTube.