Both pizzas have same size.
The first pizza is cut into 6 slices of equal size.
Each slice of first pizza = 1/6 of pizza
The second pizza is cut into 4 slices of equal size.
Each slice of second pizza = 1/4 of pizza
1/4>1/6
So slice of second pizza is greater than that of first pizza.
Part of pizza Mark got when he pick 2 Slices from the first pizza = 2×1/6 = 1/3 of pizza
Part of pizza Mark got when he pick 1 slice from each pizza = 1/6 + 1/4
= (2+3)/12
= 5/12 of pizza
For comparing 1/3 and 5/12 we need to make the denominator of both numbers equal.
To make denominator equal we have to multiply 1/3 from 4/4,
1/3×4/4 = 4/12
4/12<5/12 (when the denominator of two numbers is same the number with greater numerator is greater)
So mark was right.