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To calculate Time to Burn (TTB) with sunscreen, you would multiply the Sun Protection Factor (SPF) of the sunscreen by the Time to Burn without sunscreen. This is because the SPF indicates the level of protection provided by the sunscreen compared to not wearing sunscreen at all. By multiplying the SPF by the Time to Burn without sunscreen, you can determine how much longer you can stay in the sun without burning when wearing the sunscreen.

Now, looking at the other options:

B. TTB (without) = SPF x TTB (with) - This equation is the opposite of what we need to calculate the Time to Burn with sunscreen.

C. TTB (with) = TTB (without) / SPF - Dividing the Time to Burn without sunscreen by the SPF does not provide the correct calculation for the Time to Burn with sunscreen.

D. TTB (without) = TTB (with) / SPF - This equation is also the reverse of what is needed to calculate the Time to Burn with sunscreen.

Therefore, option A is the correct choice for calculating the Time to Burn with sunscreen.