Solve

The price elasticity of demand is positive in this case( usually it is negative ) which means you are dealing with a giffen good or a veblen good. now price elasticity of demand is % change in quantity /% change in price

demand changed from 20 to 15 % change is ((15-20)/2)/((15+20)/2)*100 = -100/7

let the price be p. percentage change in price is ((10-p)/2)/((10+p)/2)*100 = (10-p)/(10+p)*100

therfore (p+10)/(7(p-10)) = 0.71

solve for p and you will get 15.03(aprroximately 15)

for more information on giffen or veblen good see the following links

https://en.wikipedia.org/wiki/Veblen_good

https://en.wikipedia.org/wiki/Veblen_good