Q. A digits of a 2 digit no are multiplied,the digits of the product is again multiplied and this process continues untill the number becomes 1 digit number.How many such numbers are there between 10-99 which have their last result,means the 1 digit number which comes after continuous multiplication of digits, equal to a prime number?

• 10-19 12,13,15,17(4)
  20-29 21,26 (2)
  30-39 31,34,35,37 (4)
  40-49 43 (1)
  50-59 51,53,57 (3)
  60-69 62 (1)
  70-79 71,73,75 (3)
  80-89 & 90-99 none
  total=4+2+4+1+3+1+3=18
• 10 years agoHelpfull: Yes(16) No(0)
• 27 is nt in option..28 is the answer..as we know single digit prime no is 2 3 5 7
  so scanning 11=1, 12=2 13=3 14=4 15=5 16=6 17=7 18=8 19=9 20=2 (5 prime no)
  similarly 21=2 22=4 23=6 24=8 25=1 26=2 27=4 28=6 29=8 30=3 (3 prime no)
  31=3 32=6 33=9 34=2 35=5 36=8 39=4 40=4(3 prime no).41=4 42=8 43=2 44=6 45=2 46=8 47=6 48=6 49=8 50=5(3 prime no) 51=5 52=1 53=5 54=2 55=1 56=3 57=5 58=4 59=2 60=6(6 prime no) 61=6 62=2 63=8 64=8 65=3 66=8 67=8 68=6 69=2 70=7(4 prime no)71=7 72=4 73=2 74=6 75=5 76=8 77=8 78=3 79=8 80=8(4 prime no) 81=8 82=6 83=8
  84=6 85=4 86=6 87=3 88=8 89=4 90=9(1 prime no) 91=9 92=8 93=4 94=8 95=2 96=2 97=8 98=4 99=8(2prime no)
  
  
• 10 years agoHelpfull: Yes(3) No(0)
• 18 is the answer
• 10 years agoHelpfull: Yes(2) No(0)
• Prime numbers are 2,3,5,7.
  To get 2 as the last digit:
  21== 2*1=2
  but u can also split 21 as 37== 3*7=21 == 1*2=2
  similarly 73
  12== 1*2=2
  similar to abv: 12 is split as 6*2 == 62
  and thus even 26,43,34
  
  so for 2:12,26,62,43,34,21,37,73
  to get 3: 13 and 31
  to get 5: 15,35,53,51,75,57
  for 7: 17,71
  
  ans is 18.
• 10 years agoHelpfull: Yes(2) No(0)
• @parul,gr8 work girl...
• 10 years agoHelpfull: Yes(1) No(0)
• Ans is 27.,.,.anybody rem the other option
• 10 years agoHelpfull: Yes(0) No(2)
• Could you please explain it ?
• 10 years agoHelpfull: Yes(0) No(0)
• @parul 55=5*5=25=5*2=10=1*0=0....(n u have taken as 1..)aren't u splendid..heheheh srry stupid..(lolz..
• 10 years agoHelpfull: Yes(0) No(0)
• @parul tune to 20, 30, 40,50 52 sabhi le rakha hai al of dem are 0 n 0 nt a prime no..
• 10 years agoHelpfull: Yes(0) No(0)
• @ROHITASHWA leave tht bro...@Parul improve your answer..
  
• 10 years agoHelpfull: Yes(0) No(0)
• 11,12,13,15,17,21,31,51,71
  ans is 9 .....correct me if i am wrong
• 10 years agoHelpfull: Yes(0) No(0)
• @ rakesh ..u r genius bro...most of ur r correct with clear explaination ..it really helps a lot..
• 10 years agoHelpfull: Yes(0) No(0)
• 10-19 11,12,13,15,17(5)
  20-29 21,26(2)
  30-39 31,34,35,37(4)
  40-49 43(1)
  50-59 51,53,57(3)
  60-69 62(1)
  70-79 71,73,75(3)
  
  total=5+2+4+1+3+1+3=19
• 10 years agoHelpfull: Yes(0) No(2)