Cost of a tubelight = t and cost of a bulb = b.
t + b = 52
cost of bulb drops by 20% and cost of tubelight increases by 50%
⇒1.5t + 0.8b = 50
Solving the two equations in t and b
t + b = 52.......(i)
1.5t + 0.8b = 50.......(ii)
Multiplying (i) equation by 4 and (ii) equation by 5
4t + 4b = 2087.5t + 4b = 250
Subtracting
3.5 t = 42
⇒ t = 12
Thus, the cost of a tube light = Rs. 12