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Hi,
Let x represent kilos of Pork and (15-x) kilos of bangus
Question states:
buy at least 15 kilos of bangus and pork
& spend no more than Php 4000
270x + 170(15-x)  ≤  4000  
For ex:
270 - 170x ≤ 4000 -2550
100x  ≤  1450
x ≤ 14.5  kilos of Pork  and kilos of bangus ≥ .5kilos
maximum kilos of Pork one can buy is 14.5 kilos  
the less pork one buys, the more bangus one can purchase
Wish You the Best in your Studies.