Question 1092567
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If  {{{5/6}}}  of the value is 15,  then {{{1/6}}} of the value is  {{{1/5}}} of 15, i.e. 3.

 
If  {{{1/6}}}  of the value is  3,  then the value itself is  6 times  3, i.e. 18.


Thus the fifth-grade chorus consists of  18 students,  and  18-15 = 3 of them are boys.
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It is a good exercise in mental calculations.