Solve 390=5*30+30left(30-1right)/2d

$$390 = 5 \cdot 30+ \frac{ 30 \left( 30-1 \right) }{ 2 } d$$

Solve for d

$$d=\frac{16}{29}\approx 0.551724138$$

Steps for Solving Linear Equation

Multiply both sides of the equation by 2.

$$780=10\times 30+30\left(30-1\right)d$$

Multiply 10 and 30 to get 300.

$$780=300+30\left(30-1\right)d$$

Subtract 1 from 30 to get 29.

$$780=300+30\times 29d$$

Multiply 30 and 29 to get 870.

$$780=300+870d$$

Swap sides so that all variable terms are on the left hand side.

$$300+870d=780$$

Subtract 300 from both sides.

$$870d=780-300$$

Subtract 300 from 780 to get 480.

$$870d=480$$

Divide both sides by 870.

$$d=\frac{480}{870}$$

Reduce the fraction \frac{480}{870} to lowest terms by extracting and canceling out 30.

$$d=\frac{16}{29}$$