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Assuming that R1 is the resistor from Vcc to pin 7 and R2 is between pins 7 and 6, you cannot have t1<t2 with the basic astable 555 configuration. This is because t1 is determined by R1+R2 while R2 is set by R2 alone so that t1 is always greater than t2. Other configurations can give results with t1 less than t2, but to get a high ratio of 1:20, the required values are too critical to be practicable.
One thing you can do is to make t1 = 20ms and t2 = 1ms and then invert the output with a transistor or a logic gate.
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