It would take them 7.1 hours to charge the same amount
The equation for Singer A is:
80x + 14.25
The equation for Singer B is:
92 + 12.55x
x would be the number of hours
To figure out when they would charge the the same amount, set them equal to each other:
80 + 14.25x = 92 + 12.55x
Subtract 80 to both sides because whatever you do on one side you have to do it on the other side
14.25x = 12 + 12.55x
Subtract 12.55x
1.7x = 12
Divide by 1.7
x = 7.0588
Which is also
x = 7.1 hours