Explanation:
the area to be covered is
12m × 15m = 180m²
8kg of cement are needed for 10m².
180m² is 18 times 10m².
so, we also need 18 times the amount of cement :
8 × 18 = 144kg
144kg of cement are needed.
FYI : formally this is done via proportions (ratios) :
we know for the given example we have the ratio
8kg / 10m² = 4kg / 5 m²
any cement to area situation must follow the same ratio :
x = kg is cement needed
4/5 = x/180
4×180/5 = x
4×36 = x
144 = x