a)
[tex]( \frac{1}{2} - \frac{2}{5} ) \times 6 \frac{2} {3} = ( \frac{5}{10} - \frac{4}{10} ) \times \frac{20}{3 } = \frac{1}{10} \times \frac{20}{3} = \frac{2}{3} [/tex]
b)
[tex]4 \times 3 \frac{1}{8} - (2 \frac{1}{2} ) {}^{2} = \frac{4}{1} \times \frac{25}{8} - \frac{5}{2} \times \frac{5}{2} = \frac{25}{2} - \frac{25}{4} = \frac{50}{4} - \frac{25}{4} = \frac{25}{4} = \frac{25}{4} = 6 \frac{1}{4} [/tex]