We know the relation
#v=nxxlambda#
Where
#n->"frequency of wave"#
#lambda->"wave length of wave"#
#v->"velicity of wave"#
Velocity of wave being constant we can say
#n_1xxlambda_1=n_2xxlambda_2.....[1]#
Where
#n_1->"initial frequency"#
#lambda_1->"initial wave length"#
#n_2->"changed frequency"#
#lambda_2->"changed wave length"#
By the given condition
#n_2=50%xxn_1=n_1/2#
So by relation #[1]
#n_1xxlambda_1=n_1/2xxlambda_2#
#=>lambda_2=2lambda_1#
Percent increase in wave length
#=(lambda_2-lambda_1)/lambda_1xx100%#
#=(2lambda_1-lambda_1)/lambda_1xx100%=100%#
