let c be the number of pounds of cashews
then he has a total of (30+c) pounds of cashew/peanut mixture
If he sells all of it he will have 3(c+30) dollars
If he sells the peanuts and cashews separately he will have 1.50(30) = 45 dollars for the peanuts and 5c dollars for the cashews.
Set those 2 amounts equal to each other
3(c+30) = 45 + 5c
3c + 90 = 45 + 5c
45 = 2c
22.5 = c
The answer is 22.5 pounds of cashews