Let g = original # of girls at the assembly.
Let w = # of boys at the assembly.
Equation of ratios:
w          3     boys
------- Â Â = Â Â ---- = --------------------------------------------
g-12 Â Â Â Â Â Â 4 Â Â Â girls after 12 have left assembly
                                           4w + 36
Cross-mult., 4w = 3g - 36, so 4w + 36 = 3g, and g = --------------- Â (answer)
                                              4
This is "original # of girls at the assembly in terms of w, the # of boys at the assembly."