Let us assume cost of the tricycle be x.
It is given that it is sold at a gain of 15% = 15x/100.
Also it is given that if it sold for 108 the profit would have been 20% = 20x/100.
20x/100 - 15x/100 = 108
5x/100 = 108
5x = 10800
x = 2160.
The cost price of the tricycle = 2160.