Set it up as a function:
Y = 8 - (7/10)x
Y = 6 - (1/5)x
Y is the height.
Set the equations equal to each other and solve for x.
8 - (7/10)x = 6 - (1/5)x
8 - (5/10)x = 6
- (1/2)x = 6 - 8
-(1/2)x = -2
x=4 hours
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