Half of the 20 customers, which is 1/2*20=10 bought 20 bowls each, receiving 2 for every 10.
The total number of reward bowls each customer who bought 20 bowls received is 2+2=<<2+2=4>>4
If 10 customers bought 20 bowls, Howard gave away 4*10=<<4*10=40>>40 bowls as rewards.
The total number of reward bowls that remained is 70-40=<<70-40=30>>30
#### 30