Using their app from Playstore named Google opinion rewards. You can sign up and answer quick surveys on the application and get rewarded with Google play credits. These Google play credits can be exchanged to Paypal or google play gift cards to purchase paid apps on Playstore (but on some countries it can only be exchanged to google play gift cards).
Reviews on Google Play Rewards:
Download the Google Opinion rewards on playstore:
https://play.google.com/store/apps/details?id=com.google.android.apps.paidtasks
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