At his craftwork store, Howard has a collection of 70 wooden bowls where he rewards two to his customers for every 10 they buy. If he had 20 customers that day, half of whom bought 20 bowls each, calculate the number of bowls remaining in the rewards collection. Let's think first. Chain of thought:
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 = 4. If 10 customers bought 20 bowls, Howard gave away 4 * 10 = 40 bowls as rewards. The total number of reward bowls that remained is 70 - 40 = 30.
Therefore, the answer is 30.