Mrs Huang cut a cloth of length 15 m into pieces. Some pieces were 120 cm long while the rest were 150 cm long. There was 1 more piece of length 150 cm than of length 120 cm. How many pieces of length 150 cm did Mrs Huang have?
Since 1 m is equal to 100 cm, then 15 m is equal to 15 x 100 = <<15*100=1500>>1500 cm.
Let x be the number of pieces of length 150 cm.
So, the number of pieces of length 120 cm is x - 1.
The total length of the pieces of length 150 cm is 150 cm x x = <<150*x=150x>>150x.
The total length of the pieces of length 120 cm is 120 cm x (x - 1) = <<120*(x-1)=120x-120>>120x - 120.
Thus, 150x + 120x - 120 = 1500.
Combining like terms, we get 270x - 120 = 1500.
Then 270x = 1500 + 120 = 1620.
Therefore, x = 1620/270 = 6.
Therefore, Mrs Huang had 6 pieces of length 150 cm. Answer: \boxed{6}.