M&B
This is one of the annoying mass and balance ones where they give you alot of information and some which you dont need.
The numbers you need are the ground supports, 20cm x 200cm. So changing to metres this is 0.8 metres squared.
Having a quick look at cap696, your max loading intensity is 68kg per square foot. So start thinking about conversions... 1 metre = 3.28feet
So a foot squared is 3.28 x 3.28 = 10.76. So you now know you have 10.76 square feet in a square metre.
In the question we dont have a square metre, just 0.8 of a square metre, so you want to do 0.8 x 10.76, which gives you 8.61 square feet.
So your ground supports are 8.61 square feet, but your max load intensity is 68kg per square foot, so your max load is 68 x 8.61, giving you 583.3kg.
You only have 300kg loaded so far therefore you can add 285.3kg and still remain within your max load intensity limits.
Hope that helps! I always found some of the mass and balance questions like this one easier if you make a little sketch to see whats actually going on, and from there taking the figures that you need to go on and finish the question.