Calculating profit

Profit is how much you gain from a sale. In dollars, it is simply the selling price minus the cost price.
If something costs $70$ to buy and sells for $120$, the profit is $120 - 70 = 50$ dollars.

To turn that into a profit percent, divide the profit by the cost price and multiply by $100$. It is important to divide by the cost, not the selling price.
So the profit percent here is $\frac{50}{70} \times 100 \approx 71\%$.

If the selling price is below the cost price, the profit comes out negative, which is a loss. It is worked out the same way.

A shop buys a bag for $40$ dollars and sells it for $60$. What is the profit?
Profit $=$ selling price $-$ cost price.
$60 - 40 = 20$ dollars.

The same bag cost $40$ and gave a profit of $20$. What is the profit percent?
Divide the profit by the cost price and multiply by $100$:
$\frac{20}{40} \times 100 = 50\%$.

A watch is bought for $200$ and sold for $170$.
The profit is $170 - 200 = -30$, a negative number, so it is a loss of $30$ dollars.
As a percent: $\frac{-30}{200} \times 100 = -15\%$.