solution:
let the required the number of days be x
less person,more days( indirect proportional)
less hours ,more days(indirect proportional)
more works more days( direct proportional)
persons 14:15::21:x
hours per day 6:8::21:x
work y:3/2y::21:x
14×6yx=15×8×3/2y×21
x=15×8×3y×21/2×14×6y
x=45