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工厂用铁铸造零件时,先要用木料制成一实心的与待铸造零件大小及形状完全相同的模型,用这个木质的模型制造出一个模具,再将熔化的铁水倒入这个模具中,就可以铸造出所需要的零件了。已知所制成的木质模型的质量m=3kg,所使用木材的密度ρ=0.6×103kg/m3,铁的密度ρ=7.8×103kg/m3,不计铁水冷却过程中体积的变化。求:
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    To control the nation's growing problem with food wastage, the South Korean government has started a special initiative (倡议) ---“Pay as You Trash”. According to the initiative, the South Korean government has three methods in place to charge citizens for the food thrown away. One is through an RFID (Radio Frequency Identification) card — when users tap this card over a specially designed food waste bin, the lid will open. This waste is automatically weighed and recorded in the user's account. The user needs to settle this bill every month.

    The second billing method is through pre-paid garbage bags. These specially designed bags are priced based on the amount of space. For instance, in Seoul, a 10-liter garbage bag costs around 190 won (less than $1). There's also a bar code management system in place, in which residents (居民) throw food waste directly into dustbins and pay for it by buying bar code stickers attached to the bin.

    Even before the pay-by-weight system was introduced, South Koreans were still being charged for food waste — the cost was simply divided equally among the residents of each apartment block. The new system is not only fair, but is also designed to make consumers really feel the pinch of too much waste. The more food they throw out, the more they end up paying.

    Thanks to the initiative, residents like Seoul housewife Ms. Kwan are now adopting creative methods to avoid food waste. She makes sure to pour all the liquid out of leftover food before throwing it away. While preparing vegetables, she tries to use as much as the eatable parts as possible in order to reduce waste.