When a boy triples his speed, his kinetic energy becomes

Given that, speed of particle is becoming 3 times of its initial value while moving. ie $v'=3v$

As we know $K.E = \dfrac{1}{2}$ $mv ^{2}$ ..........(1)

After velocity tripled $K.E' = \dfrac{1}{2}$ $m(3v) ^{2}=\dfrac92mv^2$...........(2)

From equation 1 and 2

$K.E' = 9 \times K.E$

When a boy tripled his speed, his kinetic energy will becomes nine times.