In an alloy 80% copper and rest is tin.  In another alloy copper 85% and 13% tin.  Both are mixed in which ratio such that percentage of tin becomes 15% in resultant mixture.

2 : 5

In first alloy tin percentage = 100 - 80 = 20%

In second alloy tin percentage = 13%

After mixing tin percentage = 15%

Now using allegation method with tin percentage:

Therefore, alloy be mixed in the ratio = 2 : 5