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A truck carries 15 tonnes of rigid packets, each at least 0.5 tonne, against a suitable headboard supporting the top packets by at least 150 mm. What minimum combined rated strength must the two tie downs provide?
卡车载有共15吨、每包至少0.5吨的刚性货物，紧靠合适前挡板，顶层有至少150毫米受支撑。两条捆绑装置的合计额定强度至少是多少？
The 15-tonne rigid-packet load needs combined rated lashing strength of at least 15 tonnes when a suitable headboard supports the top packets by at least 150 mm.
顶层货物有至少150毫米受合适前挡板支撑时，15吨刚性货物的捆绑装置合计额定强度至少15吨。
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Each packet must weigh at least 0.5 tonne. You cannot apply this one-times requirement if only the lower packets are supported and top-packet support is less than 150 mm.
本题按每包至少0.5吨且符合支撑条件计算；只有下层受支撑而顶层不足150毫米时，不能套用1倍要求。