5 ms·

Does anyone know how the second expression in the picture is evaluated? "12 - 3 = 15"

blahedo 11y ago
That's meant to be "12 + 3 = 15" (or maybe it's an intentional error to make the thing look more confusing?). The idea behind this particular subtraction algorithm is to reinforce two number-sense ideas: 1) a subtraction problem like "32 − 12 = ?" is equivalent to an addition problem "12 + ? = 32", and 2) we can group additions in ways that are convenient to use numbers that are easy to add or to use convenient intermediate results that are multiples of 5 or 10 or 100. EDIT: to fix my own math typo. :P
- namelezz 11y ago
  LOL, "New math" was misleading. Thanks for your explanation.
gohrt 11y ago
The illustration was copied from the corner of the Internet where innumerate wingnuts tried to discredit Common Core but ended up highlighting their own ignorance. Google search link, sorry the URL stinks: https://www.google.com/search?tbs=sbi:AMhZZis86j61bP06Kk5rlrdY2JVS0H-D1Xsn5sK08f05G1wSqOdLQ5g8T9U_1yF-w7sv38paVFKOdSVHysQNKwWbK4YDdxZm6MqKqCux3W0dwtWLAZKHw1U8MhpBvinHzxDzAyIouinbahNg99ZAqVBD1jJV-GM--c4QnTZdrhFnOqEBzoEaeHDQuDtqrAudr4p7CHLUuhVGEn-XZYm3Ns4GHlGIvrR5s8bhkocwdS0z0vxdtaZo96q6jygbSB8hh9jQuCMFrPYUY4_1I0R-D8Ie79Kx35Yyy_1MUq_1zPo38apyjf67JWzxPIdyqEmm8Vjhl_1Yn-rj_1Bm9_1JfERtUpViA9XMmn5x8S1e-Zqte1UCnXDVmpA0lUGw2GKyJ4ljKlwBblXwsGEzVbnM1mky-unilzl8UJxXE1lfTstS2StECOyckCd6Q1sribDT5_1QCLcpAbQ6HR1ZKKYk74m9UInbq9Q-oydbkgXu617TJRTkboBgzo3tdXcqEsj5EK-djbD7iCeTWggLgSKyQWZv9wragMzWzMr0N6fPIxwal-V9Aj3zCKkYHxkFXPVPqNHA9RqRqCLnmCLSHDIAL1LgPArxqL97Nc4N9O0KLgyqIkDDc-BizjT4M_1PvoIiwdiAsKiJDj_1QjoYqssEazUalAs_1jwJbj1h3k2qbF-u5ddWVuqb-Xfh98I8G_1nJjZIBsaWTBZo35aS2yT7z2N5NLbZU2GITFtnKQoZeyS0TkRivDhrVUx_1NzlMbnYZwW_1K4Wu5jLKWSgCPztn4TSLWVKfFREc2zdvVX2vsQLKmxkg1M_1mjFEoOWmmp8BHT6qC6_122vjSbT5iPbq5Nx35a-n0fIRhfDTYmcfzTWBqCO4pfp5u3lmFSpSJc4BUf4Zg6ey9o3nEMzpCynNaudsn7JcNvBq17PtZ0rwpKtChCeHoDSqnZszvf0t97tblltV2Qt35Q8Panot0ns2ko5jZBlZQQDZs6kpmo4J1P0Od4dKym-w14G54poln_1SQiIcqx4VLJbSLpkhBa4dYUnmkbYNXCWGjoczmmRR6NRYb8D--or9tvaJyjbqTDEBBzWihYEuDPKShccwNamxoxt0PqFddSAlVEC4V8vGoEyOqluHxzElKqxL3cXCYoajTT8CU1wdU7ALhrAvTX797WOqSuSWLNu8QBW3SSsz9ekMX6MOTD-gFplMBSemfYuD5QBEt_1sUD-scqHz4cwUab6Pv4JajRCUQkNBhyGmYDN8STXCNprUaXZwPEGsQTyW1GjTepCnbsKyA8Hl1TMwiCjVYP5g7QHtxu2853xJMsAEYpjoMf_1F6fvClxcDfYx8DqdPGq5JGBEQpwsCcHqUwyWI-Tzw7xni19StRMKcFlIIUo0SxUn3aSmUgV23sCFBV7pnTgzl16cKopXBlBkFPGq-nu93jfVJvLmBAVE_1ZSOhIiOsfY8eQKBEtCxeZycUJ9zMO0b5CkmwKTGj43UYYyjsercPXWaal6GwheFSgs4iND33oU7Vp8XPl4Bw121R5G6ZzdxSJ0pHEc_1x3xhy1IPV2Hp7HI-iQo8z0GQ01lYp77yr8pQ&btnG=Search%20by%20image&hl=en

That's meant to be "12 + 3 = 15" (or maybe it's an intentional error to make the thing look more confusing?). The idea behind this particular subtraction algorithm is to reinforce two number-sense ideas: 1) a subtraction problem like "32 − 12 = ?" is equivalent to an addition problem "12 + ? = 32", and 2) we can group additions in ways that are convenient to use numbers that are easy to add or to use convenient intermediate results that are multiples of 5 or 10 or 100. EDIT: to fix my own math typo. :P

namelezz 11y ago
LOL, "New math" was misleading. Thanks for your explanation.

gohrt 11y ago

The illustration was copied from the corner of the Internet where innumerate wingnuts tried to discredit Common Core but ended up highlighting their own ignorance. Google search link, sorry the URL stinks: https://www.google.com/search?tbs=sbi:AMhZZis86j61bP06Kk5rlrdY2JVS0H-D1Xsn5sK08f05G1wSqOdLQ5g8T9U_1yF-w7sv38paVFKOdSVHysQNKwWbK4YDdxZm6MqKqCux3W0dwtWLAZKHw1U8MhpBvinHzxDzAyIouinbahNg99ZAqVBD1jJV-GM--c4QnTZdrhFnOqEBzoEaeHDQuDtqrAudr4p7CHLUuhVGEn-XZYm3Ns4GHlGIvrR5s8bhkocwdS0z0vxdtaZo96q6jygbSB8hh9jQuCMFrPYUY4_1I0R-D8Ie79Kx35Yyy_1MUq_1zPo38apyjf67JWzxPIdyqEmm8Vjhl_1Yn-rj_1Bm9_1JfERtUpViA9XMmn5x8S1e-Zqte1UCnXDVmpA0lUGw2GKyJ4ljKlwBblXwsGEzVbnM1mky-unilzl8UJxXE1lfTstS2StECOyckCd6Q1sribDT5_1QCLcpAbQ6HR1ZKKYk74m9UInbq9Q-oydbkgXu617TJRTkboBgzo3tdXcqEsj5EK-djbD7iCeTWggLgSKyQWZv9wragMzWzMr0N6fPIxwal-V9Aj3zCKkYHxkFXPVPqNHA9RqRqCLnmCLSHDIAL1LgPArxqL97Nc4N9O0KLgyqIkDDc-BizjT4M_1PvoIiwdiAsKiJDj_1QjoYqssEazUalAs_1jwJbj1h3k2qbF-u5ddWVuqb-Xfh98I8G_1nJjZIBsaWTBZo35aS2yT7z2N5NLbZU2GITFtnKQoZeyS0TkRivDhrVUx_1NzlMbnYZwW_1K4Wu5jLKWSgCPztn4TSLWVKfFREc2zdvVX2vsQLKmxkg1M_1mjFEoOWmmp8BHT6qC6_122vjSbT5iPbq5Nx35a-n0fIRhfDTYmcfzTWBqCO4pfp5u3lmFSpSJc4BUf4Zg6ey9o3nEMzpCynNaudsn7JcNvBq17PtZ0rwpKtChCeHoDSqnZszvf0t97tblltV2Qt35Q8Panot0ns2ko5jZBlZQQDZs6kpmo4J1P0Od4dKym-w14G54poln_1SQiIcqx4VLJbSLpkhBa4dYUnmkbYNXCWGjoczmmRR6NRYb8D--or9tvaJyjbqTDEBBzWihYEuDPKShccwNamxoxt0PqFddSAlVEC4V8vGoEyOqluHxzElKqxL3cXCYoajTT8CU1wdU7ALhrAvTX797WOqSuSWLNu8QBW3SSsz9ekMX6MOTD-gFplMBSemfYuD5QBEt_1sUD-scqHz4cwUab6Pv4JajRCUQkNBhyGmYDN8STXCNprUaXZwPEGsQTyW1GjTepCnbsKyA8Hl1TMwiCjVYP5g7QHtxu2853xJMsAEYpjoMf_1F6fvClxcDfYx8DqdPGq5JGBEQpwsCcHqUwyWI-Tzw7xni19StRMKcFlIIUo0SxUn3aSmUgV23sCFBV7pnTgzl16cKopXBlBkFPGq-nu93jfVJvLmBAVE_1ZSOhIiOsfY8eQKBEtCxeZycUJ9zMO0b5CkmwKTGj43UYYyjsercPXWaal6GwheFSgs4iND33oU7Vp8XPl4Bw121R5G6ZzdxSJ0pHEc_1x3xhy1IPV2Hp7HI-iQo8z0GQ01lYp77yr8pQ&btnG=Search%20by%20image&hl=en