Answer: Jack.
Step-by-step explanation:
Momentum is defined as the quantity of motion.
For an object that has a mass M, and a velocity V, the momentum is written as:
P = M*V
For this problem, we know that Jack and Jill had the same speed (so they had the same velocity), let's say that this velocity is v.
Then the momentum of Jack will be:
P = 90kg*v
And the momentum of Jill will be:
P = 50kg*v
We can see that Jack will have a larger momentum, and this is because he has a larger mass than Jill, but the same velocity as her.