Well have you graphed the functions yet?
Pharmacological products must specify recommended dosages for adults and children. Two formulas for modification of adult dosages levels for young children are
Cowling's rule: y = (1 / 24) (t + 1)a
Friend's rule: y = (2 / 25)ta
where a denotes the adult dose (in milligrams) and t denotes the age of the child (in years).
(a) If a = 100, graph the two linear equations on the same axes for 0 <= t <= 12.
(b) For what age do the two formulas specify the same dosage?
I would tackle the graph plotting by taking t = 0 and t = 12 and calculate the yc and yf values as below -
t : 0 12
yc : 25/6 25.13/6 (this is < 72 so these lines must cross)
yf : 0 72
I don't think these are actually parallel although they may look so over a smaller range.