Problem 1 · The ball balance
First the small balance, then the big one: two chained equations.
Integer answer, at most 4 digitsThe picture shows a balance in equilibrium. How much does each white ball weigh? Assume that, except for the balls on the balance, nothing else weighs anything (the strings, the plates, the parts of the balance...).
Copa Cangur · SCM
Easy
Closed answer
Reasoned solution
Key idea: solve from the inside out: first the small balance on the right, then the main one.
The small balance weighs $3$ grey balls against the $30$ ball: each grey ball weighs $g = 30/3 = 10$.
The main balance weighs the left plate ($2$ white and $1$ grey) against everything hanging on the right ($3$ greys $+$ the $30$ ball $= 60$):
$$2b + 10 = 60 \;\Longrightarrow\; b = 25.$$
Answer: 25