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The weighted mean of n numbers a_1, a_2,...,a_n which we give weights w_1, w_2,...,w_n equals wa = w_1*a_1 + w_2*a_2 + ... + w_n*a_n / (w_1 + w_2 + ... + w_n). Example: a grades on week tests are 2, 5, day tests 4, 1, 2 and home work 3. A weight of week test is 3, day test 2 and home work 1. The weight mean of the grades equals wa = (3*2 + 3*4 + 2*4 + 2*1 + 2*2 + 1*3) / (3+3+2+2+2+1) = 38 / 13 ≈ 2.9