"mental arithmetic" meaning in English

{
  "forms": [
    {
      "form": "mental arithmetics",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "-",
        "2": "s"
      },
      "expansion": "mental arithmetic (usually uncountable, plural mental arithmetics)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Synonym of mental calculation (“arithmetical calculations using only the human brain, with no help from any supplies (such as pencil and paper) or devices such as a calculator”)."
      ],
      "id": "en-mental_arithmetic-en-noun-8tUoueKr",
      "links": [
        [
          "mental calculation",
          "mental calculation#English"
        ]
      ],
      "synonyms": [
        {
          "extra": "(“arithmetical calculations using only the human brain, with no help from any supplies (such as pencil and paper) or devices such as a calculator”)",
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "mental calculation"
        }
      ],
      "tags": [
        "uncountable",
        "usually"
      ]
    }
  ],
  "word": "mental arithmetic"
}

{
  "forms": [
    {
      "form": "mental arithmetics",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "-",
        "2": "s"
      },
      "expansion": "mental arithmetic (usually uncountable, plural mental arithmetics)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English uncountable nouns"
      ],
      "glosses": [
        "Synonym of mental calculation (“arithmetical calculations using only the human brain, with no help from any supplies (such as pencil and paper) or devices such as a calculator”)."
      ],
      "links": [
        [
          "mental calculation",
          "mental calculation#English"
        ]
      ],
      "synonyms": [
        {
          "extra": "(“arithmetical calculations using only the human brain, with no help from any supplies (such as pencil and paper) or devices such as a calculator”)",
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "mental calculation"
        }
      ],
      "tags": [
        "uncountable",
        "usually"
      ]
    }
  ],
  "word": "mental arithmetic"
}