let's x1, x2, x3,... x49 are the edges of mst
so x1+x2+... +x49 = 800
but weight of every edge is decreased by 10,but that doesn't change the mst edges, so new edges are x1-10, x2-10, x3-10... x49-10
so we will remove 10 from both sides.... That will do 49 times
New mst weight is x1-10 + x2-10 +... + x49-10 = 800 - 490
New weight is 310