Mary and dan went to a bookstore. mary bought 2 notebooks and 3 pens for $14.50. dan bought 1 notebook and 2 pens for $7.50. how much does one notebook cost?

Let be the cost, respectively, of a notebook and a pen. If you buy 2 notebooks and 3 pens (which we know is a cost of $14.50), you spend

Similarly, if you buy 1 notebook and 2 pens (which we know is a cost of $7.50), you spend

These two equations form the following system:

Multiply the second equation by 2:

Subtract the first equation from the second:

So, a pen costs 0.5 dollars. Plug this equation in the second equation:

hello, there is a new name in town and that's the human spider!

answer: d.) the total number of protons and neutrons is 15

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your friendly neighborhood human spider! .

600,000,000,000, or 6e14

step-by-step explanation: