Zequals and the Art of Estimation

In this Numberphile video Rob Eastaway presents Zequals which can be useful if you need to compute a quick estimate, for example to find out if your calculation is on the right track. The idea is to ruthlessly round every number to just one significant digit. For example: 3 * 7 = 21 zz 20 7 * 8 = 56 zz 60 436 * 68 zz 400 * 70 = 28000 zz 30000 The actual answer is 29648 and surprisingly close to the zequal result of 30000.
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