Basically, how this question looks like in numerical form is:
39.1 - (33 1/3% × 42.12)
33 1/3% can be changed to 1/3. {For e.g., 1% can be changed to 0.01 or 1/100}
Thus,
39.1 - (1/3 × 42.12)
= 39.1 - (42.12 ÷ 3)
= 39.1 - (14.04)
{Break the number up if you are doing mental sums}
= 39.1 - 10.00 - 4.00 - 0.04
= 29.1 - 4.00 - 0.04
= 25.1 - 0.04
= 25.06
Hope this helps! :)