Question 1034327
This is a problem in inverse variation, i.e., a relationship between two variables such that an increase in one causes a corresponding decrease in the other, or a decrease in one causes a corresponding increase in the other.  
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n1/n2 = d2/d1=
13/10 = x/6 Cross multiply:
13*6 = 10*x
78 = 10x
x = 7.8 months
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John