Area of the base:
We know its a square, so
10*10=100 in^2
Area of the sides:
The base is equal to a side of the square.
The formula for area of a triangle is bh/2.
10*15/2= 75 in^2
Multiply by 4 since there are 4 triangular sides of equal area.
75*4=300 in^2
Add the values
100+300=400
Final answer: C