In a restaurant, the probability that a customer orders steak is 4/10. the probability of the same customer ordering dessert after their meal is ¼. what is the probability that a random restaurant customer will order dessert given that they ordered steak?

The probability of two mutually independent events happening one after the other is given by the product of both probabilities. In other words, if there is an x/100 chance of A happening and a y/100 chance of B happening, then the chance of both A and B happening is (x/100)(y/100). 
There is a 4/10 chance that a random customer will order steak. Of that 4/10, there is a 1/4 chance that they will order dessert. So, the probability that both will happen is (4/10)(1/4) = 1/10.