After 4 hours, one has travelled 12 km and the other has 8 kms.
Now if we do a little reconstrunction and drop a perpendicular from A on BC extended, we have a right angled triangle, with perpedicular, BC extended and AB as hipotenuse. AB is needed now.
length of perpedicular = AC sin 60 = 12sin60 = 6sqrt3
Length of BC extended = 8+ACcos60 = 8 + 4 = 12.
Now length of AB = Length of hipotensuse.
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