This is Sylvester's Coin Problem, also known as the Frobenius Coin Problem. Sylvester showed that the largest sum that cannot be made using coins of values a and b (where a and b are coprime) is $\displaystyle (a-1)(b-1)-1 = ab - a -b$.

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In the specific case of coins of value 6 and 11, note that 50 can be made up as 4.11 + 1.6 and then 51=3.11+3.6, 52=2.11+5.6, 53 = 1.11 + 7.6, 54 = 0.11 + 9.6, 55=5.11 + 0.6, 56 = 4.11 + 2.6 and the pattern repeats with an interval of 6.