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Short Cut That My Teacher Taught Me :)
0.1 repeating has 1 digit repeating after the decimal point, so there is one digit in the denominator. Put the repeating digit in the numerator place. That's your answer. You can do the same to any repeating decimal: 2 digits after the point has 99 as the denominator, 3 digits is the repeating number over 99, and so on.