I don't know what you mean by "substituting the 70 from 150"...? Did you maybe mean "subtracting"...?
How many of each sort of coin would you have, if you had equal amounts of each?
How many of each sort would you have, if you traded five dimes for five nickels? (That is, if you trade five coins, how big is the difference in the numbers of coins?)
How many of each sort would you have, if you traded ten dimes for ten nickels? (That is, if you trade ten coins, how big is the difference in the numbers of coins?)
Can you figure out from that how many dimes you need to switch for nickels?
Eliz.