A rectangular vegetable field is 280 meters long and 120 meters wide. If an automatic rotary sprinkler with a range of 10 meters is used to water it, how many such sprinklers are needed? (Note. You cannot divide by area)
一块长方形的蔬菜地，长280米，宽120米，如果用射程是10米的自动旋转喷灌装置浇水，大约要多少个这样的喷灌装置？(注意．不能用面积相除)
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