If 11 times a number is added to twice the number, the result is 78.

x = 6

Step-by-step explanation:

The mechanic who worked 5 hours charged $115 per hour, the one who worked 15 hours charged $120 per hour.

Step-by-step explanation:

x = rate of mechanic who worked 5 hours

y = rate of mechanic who worked 15 hours

Then

x + y = 235

5x + 15y = 2375

Just solve these 2 equations above for x and y.

x + y = 235 ⇒ x = 235 - y

Substitute into second equation to find y.

5(235 - y) + 15y = 2375

1175 - 5y + 15y = 2375

10y = 1200

y = 1200/10 = 120

x = 235 - y = 235 - 120 = 115