7 The table sets out the demand and supply schedules for chewing gum.

| Price (cents per packet) | Quantity demanded (millions of packets per week) | Quantity supplied (millions of packets per week) |

|---|---|---|

| 20 | 180 | 60 |

| 30 | 160 | 80 |

| 40 | 140 | 100 |

| 50 | 120 | 120 |

| 60 | 100 | 140 |

| 70 | 80 | 160 |

| 80 | 60 | 180 |