$a)$

$p\cdot 6,4=2,88$       $\mid :6,4$   

$p=2,88:6,4=288:640=\frac{288}{640}=\frac{2\cdot 144}{2\cdot 320}=\frac{144}{320}=\frac{2\cdot 72}{2\cdot 160}=\frac{72}{160}=\frac{36}{80}=\frac{9}{20}=\frac{45}{100}=0,45$