If it takes 20 h to convert 100 g of reactant A to product C at 20 degrees Celcius, approximately how many minutes would it take at 90 degrees Celcius? (a formula for this would be so helpful) Thanks!

You approximately double the rate of reaction for each 10 degrees C rise in temperature. How many 10s do you have from 20 C to 90 C? 2^7?