I read this as 14 2/7% fall in price=100/7 %=100/700=1/7. Let s=original selling price of one banana. 6s/7 is the new selling price.
Cost of x bananas before the price reduction is given by xs=420=(x+10)6s/7 where 6s/7 is the price after reduction.
Therefore xs=(x+10)6s/7 so x=6(x+10)/7. 7x=6x+60, x=60. Therefore s=420/x=tk7.
The reduced price of a banana is 6/7 of 7=tk6. 12 bananas cost 12*6=tk72.
CHECK: the cost of 60 bananas at tk7 each=tk420; the cost of 70 (i.e., 10 more) bananas at the reduced price of tk6 each=tk420.