Mary works as an english tutor. on monday, she earned $21.60. each day, her daily earnings increase by $1.50. how much will mary earn on thursday of the same week?

Mary works as an English tutor. On Monday, she earned $21.60. Each day, her daily earnings increase by $1.50. How much will Mary earn
on Thursday of the same week?

First of all, you gotta highlight the key things in the question so you know what to do :). 
Secondly, you add both numbers together because it says HOW MUCH so, $1.50 + $21.60 = $23.1 or $23.00.

Therefore, Mary earns $23.00 on Thursday of the same week !! ^_^

Hope this helps ^_~