a) Ta có: 2a = 5b
=> $\frac{a}{5}=\frac{b}{2}$
Lại có: $\frac{a}{5}=\frac{3a}{15}; \frac{b}{2}= \frac{4b}{8}$
=> $\frac{3a}{15} = \frac{4b}{8} = \frac{3a+4b}{15+8} = \frac{46}{23}=2$
=> 3a = 2. 15 = 30 => a = 10
4b = 2. 8 = 16 => b = 4.
b) a : b : c = 2 : 4 : 5
=> $\frac{a}{2}=\frac{b}{4}=\frac{c}{5}=\frac{a+b-c}{2+4-5}=\frac{3}{1}=3$
=> a = 2. 3 = 6
b = 4. 3 = 12
c = 5. 3 = 15