The cost of the horse given the amount received at the end of 6 months is 600
let
6/12 × 1200 + 6h/12 = 400 + h
700 + 6h / 12 = 400 + h
700 - 400 = h - 6h/12
300 = 12h - 6h / 12
300 = 6h/12
300 × 12 = 6h
3600 = 6h
h = 3600 / 6
h = 600
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