Calculating the end-to-end delay of a message sent over a network
In this case the propagation delay is more than the transmission delay, the propagation delay acts as a bottleneck for the whole network therefore the total time depend on it.

Consider a pizza shop, it takes 10 min to prepare pizza but 5 min to transport it, having infinite labour to transport, calculate how much time will it take to finsh job for 100 pizza.

Your ans might be correct but equation is wrong, you should draw the transmission diagram for better understanding.