Musa is the class teacher of a class of 45 students. He wants to split them into three groups by age. If a third of the class is under 11 years, and two-fifths are above 11 but under 13, how many students will be in the third group (13 years and above)?
The first group is a third of the class which is (1/3)*45 = 
<gadget id="calculator">1/3*45</gadget>
<output>15</output>
15 students.
There are 15 students in the first group, so there are 45-15 = 
<gadget id="calculator">45-15</gadget>
<output>30</output>
30 students.
The second group is two-fifths of the class which is (2/5)*45 = 
<gadget id="calculator">2/5*45</gadget>
<output>18</output>
18 students.
There are 18 in the second group so there are 30-18 = 
<gadget id="calculator">30-18</gadget>
<output>12</output>
12 students left in the third group.

<result>12</result>