Alright, lets get started.
1 dozen, means 12
Collecting 1 dozen marbles means 12 marbles.
Nelson has 7 whole dozens means = 7 * 12 = 84
And he has 5/12 dozen that means = 5 / 12 * 12 = 5
So, in total = 84 + 5 = 89
Means Nelson has 89 marbles in his collection. Answer
Hope it will help :)