How do I solve problems like the one in the description?

Question

How do I solve problems like "Joe bought 9 oranges, 4 pears, and 30 grapes, he spent in all of them $ 9.50.

How much do pears cost if oranges cost $ 0.55 each?"

Answer 1

 

 We need either the price of the pears, the price of the grapes, or how many fruits were purchased altogether.

 

 This is because we have three unknowns, namely the cost of the oranges, the cost of the pears, and

the cost of the grapes.

 

 We are given the total cost of the fruit and how many of each was purchased. That's the first equation:

   9O + 4P + 30G = 950 ---> amount is in pennies

 

The second equation is P = 55, as oranges are 55 cents

 

We need a third piece of info to solve the problem.