Answer:
x = 50 grs
y = 50 grs
Step-by-step explanation:
" x "   is quantity of bronze of 5% tin
" y "   is quantity of bronze of 45% tin
The worker needs 100 grs of bronze and this bronze must be 25% tin
The mathematical expression for the 100 grs condition is:
x + y = 100 
And the The mathematical expression for the % condition is:
0,05*x  + 0.45*y  = 0,25 * ( x + y )
The system is:
x + y = 100 
0,05*x + 0,45*y = 0,25* ( x + y )
Solving this system we get values of x and y
y = 100 - x
0,05*x  + 0,45* ( 100 - x ) = 0,25 * ( x + 100 - x )
0,05*x  + 45 - 0,45*x = 0,25*( 100 )
0,05*x  + 45 - 0,45*x  = 25
- 0,4*x  = -45 + 25
x = 20/0,4            x = 50  grs
and y = 50 grs