Alright, so let's work it out using words, and then we can transcribe to equations. Lot's of times people have difficulties with word problems because they rush into putting in Xs and Ys without formalizing a plan first.

We are going to end up with a jar of nuts that will cost $3/pound, and contain 30 pounds total. It will be made of some amount of peanuts (we call it P pounds of peanuts) and cashews (call it C pounds of cashews).

So the first part, is kind of easy, we know that what the total weight is and what each one of the nuts contributes. We can say it like this, P peanuts and C cashews make up 30lbs of nuts. Or in other words:

P+C=30.

Now the price. How would you determine the price of the mix? Well one way is to figure out how much the total amount of peanuts will cost 30lbs*$3/lb=$90. Some of that price comes from the peanuts of which there are P and which cost $1.50 and some of it comes from cashews of which there are C and which cost $5.

So the total price (90) is the sum of the amount of peanut * unit price and the amount of cashew * unit price.

In other words 90=1.5(P)+5(C).

Solving should be simple, but if you are having trouble, just solve for C in terms of P for the first equation and plug it into the second.

These kind of conversions are easier if you write out all the units to make sure that everything balances. For example

lbs of nuts * $/lb of nuts = lbs of peanut * $/lb of peanut + lbs of cashew * $/lb of cashew

$=$+$. Which checks. This is called dimensional analysis and although it seems easy, it's something I often do with much more complicated equations in real systems to make sure I set things up properly.