The Grandfather's brain challenge

Today Briquinex takes you to celebrate the grandfather's birthday. All his grandchildren came to congratulate him. To treat his guests, grandfather picked from the garden a number of apples and 3 times more nuts. After each grandchild was served with 3 apples and 8 nuts, the grandfather was left with 20 apples and 75 nuts. The challenge for you is to determine how many grandchildren does the grandfather have?

photo source: wikipedia