1:75;
what this actually means is that any dimension on the plan is 1/75 of actual dimension of the pool or any dimension of the actual pool is 75× the dimension on the plan. But this is, in fact, a linear scale so we will have to do something when using it:
The 20cm3 volume on the plan is attained from length×width×height.
Each dimension will need to be multiplied by 75 to find the volume of the actual pool:
Vol of pool: 75^3×20 = 8437500cm^3
I believe this should be the answer