A computer makes $$4\times {{10}^{9}}$$ operations per second. How many operations does it make in $$5\times {{10}^{2}}$$ seconds? 
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$$4\times {{10}^{9}}\times 5\times {{10}^{2}}$$  $$=20\times {{10}^{11}}$$  $$=2\times {{10}^{12}}$$ 