Answer: Yes, Daniel has enough money to buy 2 pens.
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Explanation:
x = cost of one pencil
y = cost of one pen
4x = cost of 4 pencils
3y = cost of 3 pens
4x+3y = cost of 4 pencils and 3 pens
100 - (4x+3y) = amount Daniel has left = 7 dollars
100-(4x+3y) = 7 is one equation we can form
Another equation we can form is x = y+3 because "each pencil costs $3 more than each pen".
Let's plug that into the first equation and solve for y.
100-(4x+3y) = 7
100-(4(y+3)+3y) = 7
100 - (4y+12+3y) = 7
100 - (7y+12) = 7
100 - 7y - 12 = 7
-7y + 88 = 7
-7y = 7-88
-7y = -81
y = -81/(-7)
y = 11.57 is the cost of one pen
2y = 2*11.57 = 23.14 is the cost of two pens.
Since this is less than $25, this means he has enough to buy two pens. This assumes that we either ignore tax, or the tax is already included in the listed prices.
