Question 187792
b = # boys, g = # girls


" number Of Boys in a math c\lass is one less than three times the number of girls" ----> b=3g-1


"There are 39 students in the clas " ----> b+g=39


So solve system 


*[Tex \LARGE \left\{\begin{array}{c}b=3g-1 \\ b+g=39 \end{array}\right.]


to find the answer