hi can someone plz help me with this, scenario

a shop belives that sales decrease with price and that the relationship is linear, (aprox)
they estimate that they will sell 1500 bars of chocs at 25p but will only sell 700 if the price is increased to 35p.
Find the equation for sales in terms of price, how many would u expect to sell at 30p

I think the answer is 1500 / 25 = 60
1200 / 30 = 40
700 / 35 = 20

Is this right as I don't understand the term relationship is linear

Plz help me

Lindsey