He cheats his supplier by 100 grams: Suppose he buys 1100 grams for Rs 1000. (1)
And sells 900 grams at CP of 1 kg i.e. Rs 1000 (discounts etc will be added later)
Marked Price for 900 grams = 1000 + 20% of 1000 = Rs 1200
And after 10% discount, selling price for 900 grams = 90% of MP = 90 % of 1200 = Rs 1080
Cost Price of 900 grams from (1) = Rs $\frac{1000}{1100} * 900$
Thus, for 900 grams:
$1080 = \frac{100+x}{100} * \frac{1000}{1100} * 900$
x = 32%