Thanks to donations from merchants in the community, an after-school club for disadvantaged children has introduced a new promotion: those who maintain a B average for the school year will each receive $100 in gift certificates from local businesses, such as an ice-cream parlor and a clothing store. This is an example of

Positive reinforcement

Explanation:

This is an example of Positive Reinforcement

Positive reinforcement is basically a mechanism in which stimuli is reinforced through positive encouragements such as  awards, certificates, prizes, appraisal etc.

Such mementos of achievement make a person enthusiastic and also encourages them to work harder

if you use only 25 for your entertainment and only 200 for the groceries that you really need and take out the 50 per month for the computer then that should be enough