If 20a+300=b what is the value of a+15

Simba travel agency arranges trips for climbing mount kilimanjaro. for each trip, they charge an initial fee of \$100$100 in addition to a constant fee for each vertical meter climbed. for instance, the price for climbing to the shira volcanic cone, which is 30003000 meters above the base of the mountain, is \$400$400. let f(d)f(d) denote the fee of a trip ff (measured in dollars) as a function of the vertical distance climbed dd (measured in meters). write the function's formula. f(d)=