Production speed B =755= 15 pieces/minute,
Therefore, before increasing the production speed, B has produced 150 parts;
(2) A combined image can be obtained as follows:
A:Y A = 25x(0≤x≤20)；
The speed of B is 50 PCs/min, so it takes 7 minutes for B to produce 500 parts.
B: Y B = 15x(0≤x≤ 10)，
When 10 < x ≤ 17, let y b =kx+b, and substitute (10, 150) and (17,500) into10k.
Solution: k = 50b =? 350,
So Yb = 50x-350 (10 ≤ x ≤17).
To sum up, Ya = 25x (0 ≤ x ≤ 20);
Y B = 15x(0≤x≤ 10)50x？ 350( 10