In an Area Having Sandy Soil, 50 Small Trees of a Certain

Essay

In an area having sandy soil, 50 small trees of a certain type were planted, and another 50 trees were planted in an area having clay soil. Let X = the number of trees planted in sandy soil that survive 1 year and Y = the number of trees planted in clay soil that survive 1 year. If the probability that a tree planted in sandy soil will survive 1 year is .7 and the probability of 1-year survival in clay soil is .6, compute an approximation to $P ( - 6 \leq X - Y \leq 6 )$ (do not bother with the continuity correction).