Part 1:
Principal=₹2000, Time=2 yrs, Interest=2%
Amount=P*R*T=2000*2*2%=₹80
Total=₹2000+₹80=₹2080
Part 2:
Principal=₹1000, Time=2 yrs, Interest=5%
Amount=P*R*T=1000*2*5%=₹100
Total=₹1000+₹100=₹1010
Part 3:
Total to be returned=₹2080+₹1010=₹3090
Money returned=₹2800
Cost of watch =₹3090-₹2800=₹290
Therefore Cost Of Watch is ₹290.
3090 is written because he must return 2000 rupees+its intrest and 1000 rupees and its intrest
sum of them is 3090
it is explained above
please try to understand