La estación de tren se abre a las 7 de la mañana, y tres líneas de trenes A, B y C han salido a la misma hora. El tren A pasa cada 8 minutos por la estación y el tren B pasa cada 20 minutos. El tren C pasa cada media hora una vez. ¿A qué hora volverán a pasar los tres trenes a la vez por la estación?

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You need to figure out at which x-value does the graph has a tangent line slope of -1.remember, the derivative is the slope of the tangent line. so find the derivative function and set it to equal -1 to find the x-value that will have a tangent line slope of 0.differentiate with the chain rule.    set the derivative to equal -1 and solve for x.      take log base 10 of both sides. also recall that  .    so the x-coord of the point of tangency is .the y-coordinate of this point of tangency can be calculated using the original function:     using point-slope form, the equation of the tangent line is      the x-intercept of this tangent line is when the graph is at y=0.    this rounds off to  .choice c.