Ketan sold 151 chairs

Let Cost Price (CP) of one chair = ₹1
Let Selling Price (SP) of one chair = ₹x
Total CP of 151 chairs = $151$
Gain = SP of 51 chairs = $51x$
Total SP of 151 chairs = $151x$
Profit = Total SP − Total CP
$151x - 151 = 51x$
$151x - 51x = 151$
$100x = 151$
$x = 1.51$
Profit per chair = $1.51 - 1 = 0.51$
Profit %
$= \frac{0.51}{1} \times 100 = 51%$
Answer = 51%