With this given information that 8 filters = $39.92. What you can do is divide $39.92 by 8 and get the individual price of 1 filter. So, $39.92/8 = $4.99 (price of 1 filter). The question is asking for the cost of 6 filters so you can multiply $4.99 with 6 filters and get $29.94. The answer is $29.94 for 6 filters.