. A pen and 12 erasers cost $17.70. A pen and 15 erasers cost $20.40. How much do 6 pens cost?

Sure, let's solve this step by step:

Step 1: Let's denote the cost of a pen as P and the cost of an eraser as E. From the problem, we have two equations:

P + 12E = 17.70 ...(equation 1)

P + 15E = 20.40 ...(equation 2)

Step 2: To find the cost of each item, we can subtract equation 1 from equation 2. This will give us:

15E - 12E = 20.40 - 17.70

3E = 2.70

So, E = 2.70 / 3 = $0.90 (This is the cost of one eraser)

Step 3: Now, we can substitute E = $0.90 into equation 1 to find the cost of a pen:

P + 12*0.90 = 17.70

P + 10.80 = 17.70

So, P = 17.70 - 10.80 = $6.90 (This is the cost of one pen)

Step 4: Finally, to find the cost of 6 pens, we multiply the cost of one pen by 6:

6 * $6.90 =$41.40

So, 6 pens cost $41.40.