Question 830187
27 divided by 15 = 1.8
If 1.8cm = 1 block, then 1.8*15 should equal 27, which it does.
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If you used your initial answer of 3, you could test it by multiplying the measuring standard (3) by 15 blocks.  15 blocks * 3 = 45.  Oops.  That's not the amount given in the problem.