help to solve in terms of x

Question

The amusement park sold ride tickets in packs of 5, 10, 15, and 20 tickets. What would a packet of 5 tickets cost if 20 tickets cost $4. Solve in terms of x. I know how to get the answer but not how to write out the equation.

Answer 1

4$ for 20 tikkets=0.2$ per tikket, so 5 tikkets=1$

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20x=4$, so x=4/20=0.2