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t in (-oo:+oo)
3*(20-(12*t)) = (8/7)*(21*t+13)-(12*t) // - (8/7)*(21*t+13)-(12*t)
3*(20-(12*t))-((8/7)*(21*t+13))+12*t = 0
3*(20-12*t)+(-8/7)*(21*t+13)+12*t = 0
3*(20-12*t)-8/7*(21*t+13)+12*t = 0
3*(20-12*t)-8/7*(21*t+13)+12*t = 0
12*t-60*t+316/7 = 0
316/7-48*t = 0
316/7-48*t = 0
316/7-48*t = 0 // - 316/7
-48*t = -316/7 // : -48
t = -316/7/(-48)
t = 79/84
t = 79/84
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