You have 2 unknowns.That's why you need 2 equations (2 sentences).
You decide the system of 2 equations and ready. :D
Here is an example: 20 kg.of product A and 18 kg.of product B cost 1 380 $.
Product B is 4 times more expensive than product A.
a=?(dollar) b=?(dollar)

[tex]\begin{array}{|l} 20a+18b=1 380 \\ b=4a (we- are- replacing) \end{array}[/tex]

20a+18.4a=1 380
92a=1 380

a=[tex]\frac{1380}{92}[/tex]=15 ;b=4a=4.15=60

a-15 (dollar);b-60 (dollar)

Hello , i want to solve a problem , thank you in advance :

20kg of product A and 18kg of product B cost 1500 (€ for example)
what's the price of 1Kg of product A and 1 Kg of product B . please show me the solving method .

thank you in advance.