tu pune sub forma de fractie
x/2+y/5+z/7=(x+y+z)/2+5+7=>(x+y+z)/14=42
x+y+z=42/14
x/2=42/14=>14x=42 * 2=>14x=84=> x=84/14=>x=6
y/5=42/14=>14y=42*14=>14y=210=>y=210/14=>y=15
z/7=42/14=>14z=42*7=>14z=294=>z=294/14=>z=21
verificare: x+y+z=42
6+15+21=42
21+21=42
42=42 (A)
