Since no table is provided, so I'll just tell you how to answer this problem and you can apply using the numbers in the table.
I'll assume that x is a missing mass of one of Andrew's rocks.
Masses of the rocks that maria collected: m1, m2, m3, m4, m5 and m6
Masses of the rocks that andrew collected: n1, n2, n3, n4, n5 and x
where m1, m2, m3, m4, m5, m6, n1, n2, n3, n4 and n5 are masses recorded in the table.
Mean of mass of maria's rocks = (m1+m2+m3+m4+m5+m6) / 6 = y Kg
Mean of mass of andrew's rocks = (n1+n2+n3+n4+n5+x)/6 = z
It is given that: y = z+0.1
If you substitute in the above two equations, the only unknown will be x. It can be easily calculated then.
