SOLUTION:
√5=2.236, √10=3.162
15/(√10+√20+√40-√5-√80)]
=[15/(√10+√(4×5)+√(4×10)-√5-√(16×5)]
=[15/(√10+2√5+2√10-√5-4√5)]
=[15/(3√10-3√5)]
=[5/(√10-√5)]
=[5(√10+√5)]/[(√10-√5)(√10+√5) ]
=[5(√10+√5)]/[(10−5)]
=[5(√10+√5)]/[5]
=(√10+√5)
= 3.162+2.236
=5.398