If the price is reduced by 5%, the price will be 100×( 1-5%)=95 yuan.
Then the original profit of each pair is 100-x yuan, and the profit is 80( 100-x)=8000-80x.
Now the profit per pair is 95-x yuan.
Price reduction 100-95=5 yuan.
So the sales volume increased by 4×5=20 pairs.
So it's 80+20= 100 pairs.
Profit100 (95-x) = 9500-100x.
So the profit increased (9500-100x)-(8000-80x) =100.
1500-20x= 100
1400=20x
x=70
A: The charge is 70 yuan.