Question

The projected sales volume of a video game cartridge is given by the function
s(p) = 3000/ 2p + a

s is the number of cartridges sold, in thousands; p is the price per cartridge, in dollars; and a is a constant. If according to the projections, 100000 cartridges are sold at 10 dollars per cartridge, how many cartridges will be sold at 20 dollars per cartridge?

Please help and show the work, the answer is 60,000 but i do not understand how.