XAT 2020 Ratio, Proportion and Variation — Previous Year Question

A shop sells bags in three sizes: small, medium and large. A large bag costs Rs.$1000$, a medium bag costs Rs.$200$, and a small bag costs Rs.$50$. Three buyers, Ashish, Banti and Chintu, independently buy some numbers of these types of bags. The respective amounts spent by Ashish, Banti and Chintu are equal. Put together, the shop sells $1$ large bag, $15$ small bags and some medium bags to these three buyers. What is the minimum number of medium bags that the shop sells to them?