13、The average number of p，q，r is 4，and average number of p，q，r，x is 5，then x=
8
．
（英文詞典：average number平均數(shù)）
分析：根據(jù)題意p，q，r的平均數(shù)是4，得出P+q+r=12，根據(jù)p，q，r，x的平均數(shù)是5，得出p+q+r+x=20，從而求出x的值．
解答：解：∵p，q，r的平均數(shù)是4，
∴P+q+r=4×3=12，
∵p，q，r，x的平均數(shù)是5，
∴p+q+r+x=5×4=20，
解得x=8．
故答案為：8．
點(diǎn)評(píng)：此題主要考查了平均數(shù)的求法，解決問(wèn)題的關(guān)鍵是P+q+r=12，以及p，q，r，x的平均數(shù)是5，得出p+q+r+x=20，這種整體換元法在數(shù)學(xué)中經(jīng)常用到．
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